V4L/DVB (4214): Make VIDEO_CX2341X a selectable build option

The cx2341x mpeg encoder module is used by cx88-blackbird, pvrusb2 and
the external ivtv driver. This patch allows for cx2341x to be selected without
having to also select cx88-blackbird. This will be needed to build the external
ivtv driver or the standalone pvrusb2 driver against kernel 2.6.18

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