avutil: Add av_timegm as a public function

avutil: Add av_timegm as a public function

This is useful, since the normal timegm function isn't a standard
function (requiring _BSD_SOURCE or _SVID_SOURCE on glibc to
be visible, and not available on e.g. windows). The widely available
function mktime uses the local time zone, which requires ugly
workarounds to handle UTC time.

Signed-off-by: Martin Storsjö <martin@martin.st>