[media] DocBook/Makefile: Remove osd.h header

The av7110 uses an OSD API. Such API is not documented at all. Also,
the osd.h API uses camelCase and some other weird stuff. Also, dvb-core
doesn't recognize it.

I don't see any good reason why we should document it. It seems better
to just let it as-is. If ever needed, it is probably better to write
a different API for dvb-core.

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