linux-v4l2: Fix build errors for older api

Fix build errors for older versions of the api where
VIDIOC_ENUM_DV_TIMINGS was defined but V4L2_IN_CAP_DV_TIMINGS was not.
I was under the impression that they were added at the same time, but
apparently i was wrong there.

Thanks to kmoore@FreeBSD.org for spotting this on FreeBSD.