Fix warning for -no-feature-temporaryfile

Change-Id: Ibb022a1fd043001f628a343dc8ae509083eb4471
Reviewed-by: Tasuku Suzuki <tasuku.suzuki@qt.io>
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/corelib/io/qlockfile_unix.cpp

@@ -115,6 +115,7 @@ int QLockFilePrivate::checkFcntlWorksAfterFlock(const QString &fn)
         return 0;
     return 1;
 #else
+    Q_UNUSED(fn);
     return 0;
 #endif
 }