common: admin socket catches exception by reference

common: admin socket catches exception by reference

probably not a real bug, but resolves a compiler warning:

src/common/admin_socket.cc: In member function ‘int RaiseHook::parse_signal(std::string&&, ceph::Formatter*, std::ostream&)’:
src/common/admin_socket.cc:869:21: warning: catching polymorphic type ‘class std::invalid_argument’ by value [-Wcatch-value=]
  869 |       } catch (std::invalid_argument) {
      |                     ^~~~~~~~~~~~~~~~

Fixes: https://tracker.ceph.com/issues/63455
Signed-off-by: Casey Bodley <cbodley@redhat.com>