V4L/DVB (7900): pvrusb: Fix Kconfig if DVB=m V4L_core=y

As reported by Ingo Molnar:

x86.git testing found the following build failure:

  drivers/built-in.o: In function `pvr2_dvb_feed_thread':
  pvrusb2-dvb.c:(.text+0x127e78): undefined reference to `dvb_dmx_swfilter'
  drivers/built-in.o: In function `pvr2_dvb_adapter_exit':
  pvrusb2-dvb.c:(.text+0x128357): undefined reference to `dvb_net_release'
  pvrusb2-dvb.c:(.text+0x12836f): undefined reference to `dvb_dmxdev_release'
  [...]

with this config:

  CONFIG_VIDEO_PVRUSB2=y
  CONFIG_DVB_CORE=m

i.e. pvrusb2 is built-in, dvb-core is modular.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>