Fix warning: returning non void by using Q_UNREACHABLE

Change-Id: I08e5b629582b1286c57f6c5f991e2adaf431daa4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mitch Curtis <mitch.curtis@nokia.com>

src/corelib/tools/qdatetime.cpp

@@ -1854,6 +1854,8 @@ QTime fromStringImpl(const QString &s, Qt::DateFormat f, bool &isMidnight24)
         return QTime(hour, minute, second, qMin(qRound(msec * 1000.0), 999));
     }
     }
+    Q_UNREACHABLE();
+    return QTime();
 }
 }