fix warning in qmakevfs without PROEVALUATOR_FULL

amends 57ca8d2698.

Change-Id: Ibf56c15ab46b9dfa856dc0af7ae2e8c84caa3063
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

qmake/library/qmakevfs.cpp

@@ -65,6 +65,7 @@ bool QMakeVfs::writeFile(const QString &fn, QIODevice::OpenMode mode, bool exe,
     else
         *cont = contents;
     Q_UNUSED(errStr)
+    Q_UNUSED(exe)
     return true;
 #else
     QFileInfo qfi(fn);