vf_yadif: silence a warning.
clang says:
libavfilter/vf_yadif.c:192:28: warning: incompatible pointer types assigning to
'void (*)(uint8_t *, uint8_t *, uint8_t *, uint8_t *, int, int, int, int, int)'
from 'void (uint16_t *, uint16_t *, uint16_t *, uint16_t *, int, int, int, int, int)'