Fixes the following warnings:
libavdevice/vfwcap.c:331:35: warning: passing argument 1 of 'av_parse_video_size' from incompatible pointer type
libavdevice/vfwcap.c:331:59: warning: passing argument 2 of 'av_parse_video_size' from incompatible pointer type

Reported-by: Reino Wijnsma