Fix instance method not found warning

NSView/UIView were only forward declared here which led to warnings when
calling methods on instances of them.

Change-Id: Ic2b391bb0ed8d45306dc16e2a807ce7fcae5015e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>

src/plugins/avfoundation/mediaplayer/avfvideowidget.mm

@@ -39,6 +39,12 @@
 #include <QtGui/QPaintEvent>
 #include <QtGui/QPainter>
 
+#if defined(Q_OS_MACOS)
+#import <AppKit/AppKit.h>
+#else
+#import <UIKit/UIKit.h>
+#endif
+
 QT_USE_NAMESPACE
 
 AVFVideoWidget::AVFVideoWidget(QWidget *parent)