Fix warning: qt_setDockMenu not found

qcocoanativeinterface.mm:196:5: warning:
 instance method '-qt_setDockMenu:' not found
 (return type defaults to 'id') [-Wobjc-method-access]
    [NSApp QT_MANGLE_NAMESPACE(qt_setDockMenu): menu];

Change-Id: I80838d0098e7f172d9b8124f04e91dba64764318
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/plugins/platforms/cocoa/qcocoanativeinterface.mm

@@ -46,6 +46,7 @@
 #include "qcocoamenubar.h"
 #include "qmacmime.h"
 #include "qcocoahelpers.h"
+#include "qcocoaapplication.h"
 
 #include <qbytearray.h>
 #include <qwindow.h>