xmltv: ignore all unused fields found on DTD for version 0.6.1

While it doesn't fully support all fields at version 0.6.1
with is the last one, yet to be released:

https://github.com/XMLTV/xmltv

Make sure it accepts all elements found there, in order to avoid
messages about unknown tags.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>