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>

libavdevice/v4l2.c

@@ -427,10 +427,7 @@ static void mmap_release_buffer(void *opaque, uint8_t *data)
 #if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
 static int64_t av_gettime_monotonic(void)
 {
-    struct timespec tv;
-
-    clock_gettime(CLOCK_MONOTONIC, &tv);
-    return (int64_t)tv.tv_sec * 1000000 + tv.tv_nsec / 1000;
+    return av_gettime_relative();
 }
 #endif