[media] MAINTAINERS: add DVB_USB_GL861

I have the hardware and I have already made almost all changes what
are needed during the recent few years, so lets mark it as maintained.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>