libstdc++: Use template keyword in __mapping_of alias template

This is needed to fix an error with Clang 19:

include/c++/16.0.0/mdspan:512:30: error: use 'template' keyword to treat 'mapping' as a dependent template name
 512 |         is_same_v<typename _Layout::mapping<typename _Mapping::extents_type>,
     |                                     ^

libstdc++-v3/ChangeLog:

	* include/std/mdspan (__mapping_of): Add template keyword.

Author: jwakely

libstdc++-v3/include/std/mdspan

@@ -509,7 +509,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     template<typename _Layout, typename _Mapping>
       concept __mapping_of =
-	is_same_v<typename _Layout::mapping<typename _Mapping::extents_type>,
+	is_same_v<typename _Layout::template mapping<typename _Mapping::extents_type>,
 		  _Mapping>;
 
     template<typename _Mapping>