[libc++] Explicitly mark basic_string<...>::npos with default

visibility.

This ensures that the version compiled into the library isn't
accidentally hidden.

Author: EricWF

libcxx/include/string

@@ -4232,8 +4232,9 @@ _LIBCPP_FUNC_VIS wstring to_wstring(double __val);
 _LIBCPP_FUNC_VIS wstring to_wstring(long double __val);
 
 template<class _CharT, class _Traits, class _Allocator>
-    const typename basic_string<_CharT, _Traits, _Allocator>::size_type
-                   basic_string<_CharT, _Traits, _Allocator>::npos;
+_LIBCPP_FUNC_VIS
+const typename basic_string<_CharT, _Traits, _Allocator>::size_type
+               basic_string<_CharT, _Traits, _Allocator>::npos;
 
 template <class _CharT, class _Allocator>
 struct _LIBCPP_TEMPLATE_VIS