[media] increment DVB API to version 5.6 for ATSC-MH frontend control

increment the DVB API version to 5.6 to signify support for
controlling an ATSC-MH frontend.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>