media: dvb-frontends: DUMMY_FE should depends on DVB_CORE

media: dvb-frontends: DUMMY_FE should depends on DVB_CORE

Using select for DVB_CORE doesn't work, as reported by
Randy:

	CONFIG_I2C=m
	CONFIG_DVB_CORE=y

	ld: drivers/media/dvb-core/dvbdev.o: in function `dvb_module_probe':
	dvbdev.c:(.text+0xf92): undefined reference to `i2c_new_client_device'
	ld: dvbdev.c:(.text+0xffb): undefined reference to `i2c_unregister_device'
	ld: drivers/media/dvb-core/dvbdev.o: in function `dvb_module_release':
	dvbdev.c:(.text+0x107d): undefined reference to `i2c_unregister_device'

The problem is actually caused by the dummy frontend driver,
which uses select, and it is missing an I2C dependency:

	WARNING: unmet direct dependencies detected for DVB_CORE
	  Depends on [m]: MEDIA_SUPPORT [=y] && MEDIA_DIGITAL_TV_SUPPORT [=y] && (I2C [=m] || I2C [=m]=n)
	  Selected by [y]:
	  - DVB_DUMMY_FE [=y] && MEDIA_SUPPORT [=y] && MEDIA_TEST_SUPPORT [=y]

As this is the only frontend driver using "select DVB_CORE",
change it do depends on DVB_CORE.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>