QLocale: silence fallthrough warning

clang doesn't seem to consider `Q_ASSERT(false)` as empty statement.

Pick-to: 6.7
Change-Id: If99c7d4874450d781182491ef3963f1ee96285a6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/text/qlocale_unix.cpp

@@ -265,6 +265,7 @@ QVariant QSystemLocale::query(QueryType type, QVariant in) const
         return lc_messages.createSeparatedList(in.toStringList());
     case LocaleChanged:
         Q_ASSERT(false);
+        [[fallthrough]];
     default:
         break;
     }