Workaround an ld warning on macOS

Author: vitaut

include/fmt/base.h

@@ -2631,6 +2631,7 @@ template <typename T> struct strip_named_arg<T, true> {
 };
 
 template <typename T, typename ParseContext>
+FMT_VISIBILITY("hidden")  // Suppress an ld warning on macOS (#3769).
 FMT_CONSTEXPR auto parse_format_specs(ParseContext& ctx)
     -> decltype(ctx.begin()) {
   using char_type = typename ParseContext::char_type;