Fix warning in qtextengine compilation

enableHarfbuzz() should only be defined on Mac.

Reviewed-by: Eskil
(cherry picked from commit 5ce3fbb67b79c3732fd47b296ef9421398ca520c)

Change-Id: I9ecb5db49478c3f5beb5d41cf99320f0faedce2e
Reviewed-on: http://codereview.qt.nokia.com/375
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>

src/gui/text/qtextengine.cpp

@@ -856,7 +856,7 @@ void QTextEngine::shapeLine(const QScriptLine &line)
     }
 }
 
-#if !defined(QT_ENABLE_HARFBUZZ_FOR_MAC)
+#if !defined(QT_ENABLE_HARFBUZZ_FOR_MAC) && defined(Q_WS_MAC)
 static bool enableHarfBuzz()
 {
     static enum { Yes, No, Unknown } status = Unknown;