lavd: Use av_gettime_relative()

Whenever av_gettime() is used to measure relative period of time,
av_gettime_relative() is prefered as it guarantee monotonic time
on supported platforms.

Since v4l2 is compiled only on Linux where av_gettime_relative()
is monotonic, a small wrapper function av_gettime_monotonic() is used
(should be inlined the compiler) to put emphasis on that fact.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>