Mac QTKit Video OpenForPlayback conditional

Building for versions of OSX prior to Snow Leopard
fails since QTMovieOpenForPlaybackAttribute is not
available, this patch makes this setting conditional
on build support for OS X >= 10.6.

src/QtMacVideoWindow.mm

@@ -115,7 +115,9 @@ void VideoWindow::mediaSelected(QString filename)
         NSDictionary *attributes = [NSDictionary dictionaryWithObjectsAndKeys:
             file, QTMovieFileNameAttribute,
             num, QTMovieLoopsAttribute,
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_6
             num, QTMovieOpenForPlaybackAttribute,
+#endif
             nil];
 
         movie = [[QTMovie alloc] initWithAttributes:attributes error:&error];