avfvideosrc: fix compilation with gcc

The synthesize block needs to be moved to fix a compiler error

sys/applemedia/avfvideosrc.m

@@ -137,6 +137,9 @@ didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
 
 @implementation GstAVFVideoSrcImpl
 
+@synthesize deviceIndex, doStats, fps, captureScreen,
+            captureScreenCursor, captureScreenMouseClicks;
+
 - (id)init
 {
   return [self initWithSrc:NULL];
@@ -179,9 +182,6 @@ didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
   [super finalize];
 }
 
-@synthesize deviceIndex, doStats, fps, captureScreen,
-            captureScreenCursor, captureScreenMouseClicks;
-
 - (BOOL)openDeviceInput
 {
   NSString *mediaType = AVMediaTypeVideo;