Mark variable only used in Q_ASSERT as unused

Change-Id: Idee2a4772c7c12e293838a647ed768295612f794
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qtenvironmentvariables.cpp

@@ -377,6 +377,7 @@ bool qLocalTime(time_t utc, struct tm *local)
     tzset();
     if (tm *res = localtime_r(&utc, local)) {
         Q_ASSERT(res == local);
+        Q_UNUSED(res);
         return true;
     }
     return false;