libstdc++: Add missing export for std::is_layout_compatible_v [PR120159]

libstdc++-v3/ChangeLog:

	PR libstdc++/120159
	* src/c++23/std.cc.in (is_layout_compatible_v): Export.

(cherry picked from commit 8a1f3615c01cdbf5b2d37448c8bb09a96d5e3330)
1 file changed