QGstXvImageBuffer: fix warning about class/struct mismatch

Change-Id: I98df1ce92fab69ad1200066d310aa8bd016b5aca
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/multimedia/gsttools_headers/qgstxvimagebuffer_p.h

@@ -74,7 +74,8 @@ QT_BEGIN_NAMESPACE
 
 class QGstXvImageBufferPool;
 
-struct QGstXvImageBuffer {
+class QGstXvImageBuffer {
+public:
     GstBuffer buffer;
     QGstXvImageBufferPool *pool;
     XvImage *xvImage;