tst_moc: fix some -Wunused-* warnings on MinGW

Found while inspecting a recent MingWG log file from the CI.

Amends da284ef10e0ef80776b9fc9b7bb0e6dc8d71ba63.

Pick-to: 6.6 6.5 6.2
Change-Id: Ife10118710079e60c581314fa857be42452fe810
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 56ca2595526360f98d7824d815552635f87f178f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/tools/moc/tst_moc.cpp

@@ -4279,6 +4279,8 @@ void tst_Moc::mocJsonOutput()
             return "Error waiting for diff process to finish.";
         return diffProc.readAllStandardOutput();
 #else
+        Q_UNUSED(actual);
+        Q_UNUSED(expected);
         return "Cannot launch diff. Please check allmocs.json and allmocs_baseline.json on disk.";
 #endif
     };