Fixed compilation error with clang.

Previously, trying to compile with clang would result in the error: assigning to 'objc_object *' from incompatible type 'DockIconClickEventHandler *'

src/qt/macdockiconhandler.h

@@ -6,7 +6,12 @@
 class QMenu;
 class QIcon;
 class QWidget;
-class objc_object;
+
+#ifdef __OBJC__
+@class DockIconClickEventHandler;
+#else
+class DockIconClickEventHandler;
+#endif
 
 /** Macintosh-specific dock icon handler.
  */
@@ -31,7 +36,7 @@ public slots:
 private:
     MacDockIconHandler();
 
-    objc_object *m_dockIconClickEventHandler;
+    DockIconClickEventHandler *m_dockIconClickEventHandler;
     QWidget *m_dummyWidget;
     QMenu *m_dockMenu;
 };