Fix Obj-C property usage

...I really need to go through my Objective-C reading material

plugins/mac-avcapture/av-capture.m

@@ -89,7 +89,7 @@ struct av_capture {
 	AVCaptureInputPort *port = capture->device_input.ports[0];
 
 	CMTime host_pts = CMSyncConvertTime(info.presentationTimeStamp,
-			port->clock, CMClockGetHostTimeClock());
+			port.clock, CMClockGetHostTimeClock());
 
 	frame->data[0] = addr;
 	frame->timestamp = host_pts.value;