export names only once (#3392)

names declared to be exported at the point of introduction into a namespace *must not* be (re-)declared as exported later in the TU, e.g. when they are redeclared, defined, or specialized. [module.export]/6

Drive-by fix found during module testing: add a missing `detail::` name qualification

include/fmt/args.h

@@ -59,7 +59,6 @@ class dynamic_arg_list {
   }
 };
 }  // namespace detail
-FMT_BEGIN_EXPORT
 
 /**
   \rst
@@ -230,7 +229,6 @@ class dynamic_format_arg_store
   }
 };
 
-FMT_END_EXPORT
 FMT_END_NAMESPACE
 
 #endif  // FMT_ARGS_H_