libstdc++: Fix circular dependency for bitmap_allocator [PR103381]

libstdc++: Fix circular dependency for bitmap_allocator [PR103381]

<ext/bitmap_allocator.h> includes <function>, and since C++17 that
includes <unordered_map>. If std::allocator is defined in terms of
__gnu_cxx::bitmap_allocator then you get a circular reference and
bootstrap fails when compiling src/c++17/*.cc.

libstdc++-v3/ChangeLog:

	PR libstdc++/103381
	* include/ext/bitmap_allocator.h: Include <bits/stl_function.h>
	instead of <functional>.

(cherry picked from commit 5459fa132a99e6037e5ccf1b49d617677a584ff8)