V4L/DVB: [-next] gspca: fix build for INPUT=m or INPUT=n

Handle case of GSPCA=m, INPUT=m when building gspca core;
also handle case of INPUT=n by using stubs.

drivers/media/video/gspca/gspca.c:662: error: implicit declaration of function 'gspca_input_destroy_urb'
drivers/media/video/gspca/gspca.c:668: error: implicit declaration of function 'gspca_input_create_urb'
drivers/media/video/gspca/gspca.c:2284: error: implicit declaration of function 'gspca_input_connect'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>