Add missing moves to grouping std::string member (#4704)

Author: Skylion007

include/fmt/format.h

@@ -1219,7 +1219,7 @@ FMT_API auto thousands_sep_impl(locale_ref loc) -> thousands_sep_result<Char>;
 template <typename Char>
 inline auto thousands_sep(locale_ref loc) -> thousands_sep_result<Char> {
   auto result = thousands_sep_impl<char>(loc);
-  return {result.grouping, Char(result.thousands_sep)};
+  return {std::move(result.grouping), Char(result.thousands_sep)};
 }
 template <>
 inline auto thousands_sep(locale_ref loc) -> thousands_sep_result<wchar_t> {
@@ -1968,7 +1968,7 @@ template <typename Char> class digit_grouping {
   explicit digit_grouping(locale_ref loc, bool localized = true) {
     if (!localized) return;
     auto sep = thousands_sep<Char>(loc);
-    grouping_ = sep.grouping;
+    grouping_ = std::move(sep.grouping);
     if (sep.thousands_sep) thousands_sep_.assign(1, sep.thousands_sep);
   }
   digit_grouping(std::string grouping, std::basic_string<Char> sep)