V4L/DVB (10210): Fix a bug on v4lgrab.c

v4lgrab  breaks the fputc macro on some systems, because of #defined
FILE.

Also, I also added comments because it was not at all clear that to get gspca
cameras to work with this application you need v4l1compat.

Signed-off-by: Simon Harrison <si1356@yahoo.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>