Avoid qualifying by inline namespace. Fixes fmtlib#2642.

Author: mkurdej

include/fmt/core.h

@@ -567,7 +567,7 @@ constexpr auto to_string_view(const S& s)
 FMT_BEGIN_DETAIL_NAMESPACE
 
 void to_string_view(...);
-using fmt::v8::to_string_view;
+using fmt::to_string_view;
 
 // Specifies whether S is a string type convertible to fmt::basic_string_view.
 // It should be a constexpr function but MSVC 2017 fails to compile it in