examples/filtering: remove #include vsink_buffer.h, the header does not exist

Found-by: teratorn
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/examples/filtering.c

@@ -31,7 +31,6 @@
 #include <libavcodec/avcodec.h>
 #include <libavformat/avformat.h>
 #include <libavfilter/avfiltergraph.h>
-#include <libavfilter/vsink_buffer.h>
 #include <libavfilter/vsrc_buffer.h>
 
 const char *filter_descr = "scale=78:24";