gstreamer: can optionally use libxml2

gstreamer can optionally build against libxml2, so account for it in
the dependencies.
Functionality seems somewhat reduced without it, an example being
misdetecting subtitle (.srt) files as subrip mime type instead of
subtitle itself.

A failed scenario would be:

$ gst-typefind-0.10 some.srt
some.srt - application/x-subrip

When built with libxml2:

$ gst-typefind-0.10 some.srt
some.srt - application/x-subtitle

This doesn't affect gstreamer 1.x since it dropped any use of libxml2.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>