graphics/opencv: use multimedia/v4l_compat on non-CURRENT

Since video(4) et al landed in -CURRENT, the multimedia/v4l_compat
headers conflict with <sys/videoio.h> when both are present. Support
for <sys/videoio.h> is already included for NetBSD so ensure that
this path is taken during configure on -CURRENT. Also ensure that
the Linux-style unsigned int shorthand is defined for us in addition
to OpenBSD.

PR: 297454