Cocoa QPA: Fix compilation error

The error appeared with latest clang as of Feb. 5, 2013.

Apple LLVM version 4.2 (clang-425.0.24) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.2.0

Change-Id: I8df8cccc941ac03a7a997bdd5afe095b7b6f65d3
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>

src/plugins/platforms/cocoa/qcocoawindow.h

@@ -49,7 +49,8 @@
 
 #include "qcocoaglcontext.h"
 #include "qnsview.h"
-class QT_PREPEND_NAMESPACE(QCocoaWindow);
+
+QT_FORWARD_DECLARE_CLASS(QCocoaWindow)
 
 @interface QNSWindow : NSWindow {
     @public QCocoaWindow *m_cocoaPlatformWindow;