media: pvrusb2: VIDEO_PVRUSB2 depends on DVB_CORE to use dvb_* symbols

A rand config causes this link error
vmlinux.o: In function `pvr2_dvb_create':
(.text+0x8af1d2): undefined reference to `dvb_register_adapter'

The rand config has
CONFIG_VIDEO_PVRUSB2=y
CONFIG_VIDEO_DEV=y
CONFIG_DVB_CORE=m

VIDEO_PVRUSB2 should also depend on DVB_CORE.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>