Fix MinGW build with _WIN32_WINNT<0x600, GCC>=13, model=win32

[palves]

unordered_dense.h fails to compile on Windows/MinGW with GCC >= 13 with thread model win32, and _WIN32_WINNT < 0x600 (Vista), in CFLAGS/CXXFLAGS. E.g., GDB defines _WIN32_WINNT to 0x501, and it fails to build with GCC 14.2 like so:

   CXX    ada-exp.o
 In file included from /opt/xpack-mingw-w64-gcc-14.2.0-1/x86_64-w64-mingw32/include/c++/14.2.0/shared_mutex:42,
		  from /opt/xpack-mingw-w64-gcc-14.2.0-1/x86_64-w64-mingw32/include/c++/14.2.0/memory_resource:66,
		  from /home/pedro/gdb/src/gdb/../gdbsupport/unordered_dense.h:104,
		  from /home/pedro/gdb/src/gdb/../gdbsupport/unordered_map.h:21,
		  from /home/pedro/gdb/src/gdb/gdbtypes.h:53,
		  from /home/pedro/gdb/src/gdb/expression.h:23,
		  from /home/pedro/gdb/src/gdb/ada-exp.y:40:
 /opt/xpack-mingw-w64-gcc-14.2.0-1/x86_64-w64-mingw32/include/c++/14.2.0/bits/std_mutex.h:164:5: error: '__gthread_cond_t' does not name a type; did you mean '__gthread_once_t'?
   164 |     __gthread_cond_t* native_handle() noexcept { return &_M_cond; }
       |     ^~~~~~~~~~~~~~~~
       |     __gthread_once_t
 (... snip other instances of same ...)

The problem is that unordered_dense.h includes <memory_resource>, which uses std::mutex, and thus needs to include <mutex>. (See https://quuxplusone.github.io/blog/2018/06/05/libcpp-memory-resource, "Notice that synchronized_pool_resource contains a mutex, which means that <memory_resource> effectively must include <mutex>.")

GCC 13 rewrote the win32 thread model support, and in that rewrite, support for __gthread_cond_t in std_mutex.h became conditional on _WIN32_WINNT >= 0x600.

GCC/libstdc++ also has <experimental/memory_resource> so with this patch, such a configuration takes that branch and succeeds, because <experimental/memory_resource> does not include <mutex>.

Tested with MinGW-w64 GCC 14.2, posix and win32 models.
Tested with MinGW-w64 GCC 10.1, posix and win32 models.
Tested with GNU/Linux GCC 11.4.0.