[media] drx39xxj.h: Fix undefined reference to attach function

As reported by the kbuild test robot <fengguang.wu@intel.com>:

drivers/built-in.o: In function `em28xx_dvb_init':
    em28xx-dvb.c:(.text+0x876f2c): undefined reference to `drx39xxj_attach'

That happens when CONFIG_VIDEO_EM28XX_DVB is selected, and neither
CONFIG_MEDIA_SUBDRV_AUTOSELECT or DVB_DRX39XYJ is selected.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>