Fix Warning

QtCreator (4.9) Clang Code Model (Clang 7) gives the
warning:core.h:404:11: error: an attribute list cannot appear here
core.h:116:24: note: expanded from macro 'FMT_NORETURN'

include/fmt/core.h

@@ -401,7 +401,7 @@ struct error_handler {
   FMT_CONSTEXPR error_handler(const error_handler&) {}
 
   // This function is intentionally not constexpr to give a compile-time error.
-  FMT_API FMT_NORETURN void on_error(const char* message);
+  FMT_NORETURN FMT_API void on_error(const char* message);
 };
 }  // namespace internal