V4L/DVB (4706): Do not enable VIDEO_V4L2 unconditionally

The VIDEO_V4L2 config setting is enabled unconditionally, even for 
configurations with no support for this subsystem whatsoever.  The 
following patch adds the necessary dependency.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>