cmake: suppress -Wmaybe-uninitialized warning in memstore PageSet

cmake: suppress -Wmaybe-uninitialized warning in memstore PageSet

Suppress GCC-15 false positive warning about uninitialized memory in
Page::operator delete() using placement new pattern.

GCC-15 incorrectly warns about potentially uninitialized data when
using placement new with manual memory management:

```
[409/506] Building CXX object src/os/CMakeFiles/os.dir/memstore/MemStore.cc.o
In file included from /home/kefu/dev/ceph/src/os/memstore/MemStore.h:29,
                 from /home/kefu/dev/ceph/src/os/memstore/MemStore.cc:28:
In static member function ‘static void Page::operator delete(void*)’,
    inlined from ‘void Page::put()’ at /home/kefu/dev/ceph/src/os/memstore/PageSet.h:36:41:
/home/kefu/dev/ceph/src/os/memstore/PageSet.h:83:42: warning: ‘*this.Page::data’ may be used uninitialized [-Wmaybe-uninitialized]
   83 |     delete[] reinterpret_cast<Page*>(p)->data;
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
```

The warning is a false positive: Page instances are constructed using
placement new over pre-allocated memory, requiring manual cleanup of
the underlying data array. This is the intended behavior and memory
is properly initialized. So, in this change we just silence this warning
in CMake after checking its availability.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>