[media] fix clock_gettime cross-references

Fix those warnings:

	Documentation/media/uapi/cec/cec-ioc-dqevent.rst:124: WARNING: c:func reference target not found: clock_gettime(2)

By replacing it with the right function name, using this shell script:

	for i in `find Documentation/media -type f`; do sed 's,clock_gettime(2),clock_gettime,' <$i >a && mv a $i; done

Please notice that this will make the nitpick mode to shut up
complaining about that, becasue clock_gettime is on its exclude list,
but the cross reference will be undefined until someone documents
this function at the core documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>