V4L/DVB (9977): Kbuild: fix compilation when dib7000p is not defined

dib7000p.h defines a few extern symbols when CONFIG_DVB_DIB7000P is not set. since the
header is used on more than one driver, this causes symbol duplication, as pointed by
Ingo Molnar <mingo@elte.hu>:

drivers/media/dvb/built-in.o: In function `dib7000p_set_gpio':
(.text+0x3f242): multiple definition of `dib7000p_set_gpio'
drivers/media/video/built-in.o:(.text+0xb8c1e): first defined here
drivers/media/dvb/built-in.o: In function `dib7000p_i2c_enumeration':
(.text+0x3f282): multiple definition of `dib7000p_i2c_enumeration'
drivers/media/video/built-in.o:(.text+0xb8c3e): first defined here
drivers/media/dvb/built-in.o: In function `dib7000p_set_wbd_ref':
(.text+0x3f1c1): multiple definition of `dib7000p_set_wbd_ref'
drivers/media/video/built-in.o:(.text+0xb8bfe): first defined here
  LD      drivers/net/built-in.o
make[2]: *** [drivers/media/built-in.o] Error 1

Cc: Patrick Boettcher <patrick.boettcher@desy.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>