common/dout: fix FTBFS on GCC 14

common/dout: fix FTBFS on GCC 14

The following problem has been reported by Kaleb Keithley:

```
/builddir/build/BUILD/ceph-18.2.1/src/osd/osd_types.h: In lambda function:
/builddir/build/BUILD/ceph-18.2.1/src/common/dout.h:184:73: error: call to non-‘constexpr’ function ‘virtual unsigned int DoutPrefixProvider::get_subsys() const’
  184 |     dout_impl(pdpp->get_cct(), ceph::dout::need_dynamic(pdpp->get_subsys()), v) \
      |                                                         ~~~~~~~~~~~~~~~~^~
/builddir/build/BUILD/ceph-18.2.1/src/common/dout.h:155:58: note: in definition of macro ‘dout_impl’
  155 |       return (cctX->_conf->subsys.template should_gather<sub, v>());    \
      |                                                          ^~~
/builddir/build/BUILD/ceph-18.2.1/src/osd/osd_types.h:3617:3: note: in expansion of macro ‘ldpp_dout’
 3617 |   ldpp_dout(dpp, 10) << "build_prior all_probe " << all_probe << dendl;
      |   ^~~~~~~~~
```

For details of the problem and the idea behind the fix,
please refer to the comment this commit brings to `dout.h`.

The minimized replicator that the facilitated Goldbot-based
investigation:

```cpp
namespace ceph::dout {

template<typename T>
struct dynamic_marker_t {
  T value;
  // constexpr ctor isn't needed as it's an aggregate type
  constexpr operator T() const { return value; }
};

template<typename T>
constexpr dynamic_marker_t<T> need_dynamic(T&& t) {
  return dynamic_marker_t<T>{ std::forward<T>(t) };
}

template<typename T>
struct is_dynamic : public std::false_type {};

template<typename T>
struct is_dynamic<dynamic_marker_t<T>> : public std::true_type {};

} // ceph::dout

struct subsys_t {
  template <unsigned SubV, int LvlV>
  bool should_gather() const {
    return true;
  }
  bool should_gather(const unsigned sub, int level) const {
    return false;
  }
};

static subsys_t subsys;

  do {                                                                  \
  const bool should_gather = [&](const auto cctX) {                     \
    if constexpr (ceph::dout::is_dynamic<decltype(sub)>::value ||       \
                  ceph::dout::is_dynamic<decltype(v)>::value) {         \
      std::cout << "the dynamic path" << std::endl;                     \
      return subsys.should_gather(sub, v);                              \
    } else {                                                            \
      /* The parentheses are **essential** because commas in angle      \
       * brackets are NOT ignored on macro expansion! A language's      \
       * limitation, sorry. */                                          \
      std::cout << "the static path" << std::endl;                      \
      /*return subsys.should_gather(sub, v);*/                              \
      return (subsys.template should_gather<sub, v>());             \
    }                                                                   \
  }(cct);                                                               \
  } while (0)

  if (decltype(auto) pdpp = (dpp); pdpp) /* workaround -Wnonnull-compare for 'this' */ \
    dout_impl(42, sub, v)

  if (decltype(auto) pdpp = (dpp); pdpp) /* workaround -Wnonnull-compare for 'this' */ \
    dout_impl(42, ceph::dout::need_dynamic(42), v)

int main() {
    std::random_device dev;
    std::mt19937 rng(dev());
    std::uniform_int_distribution<std::mt19937::result_type> dist6(1,6); // distribution in range [1, 6]

    int sub = dist6(rng);
    ldpp_dout("mocked out", sub);
    //ldpp_subdout("mocked out", 4, 3);
}
```

Fixes: https://tracker.ceph.com/issues/64050
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>