Fix breakage due to removing __gnu_cxx::size_t declaration

Restore the using-declaration but locally in the source file, not in the
header.

	* src/c++98/bitmap_allocator.cc: Add using-declaration for size_t.

From-SVN: r271812

libstdc++-v3/ChangeLog

 2019-05-31  Jonathan Wakely  <jwakely@redhat.com>
 
+	* src/c++98/bitmap_allocator.cc: Add using-declaration for size_t.
+
 	PR libstdc++/90682
 	* libsupc++/eh_term_handler.cc: Include eh_term_handler.h to get
 	definition of _GLIBCXX_DEFAULT_TERM_HANDLER.

libstdc++-v3/src/c++98/bitmap_allocator.cc

@@ -28,6 +28,8 @@ namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+  using std::size_t;
+
   namespace __detail
   {
     template class __mini_vector<