V4L/DVB(6548b) Fix compilation for em28xx

From: Mauro Carvalho Chehab <mchehab@infradead.org>

In file included from drivers/media/video/em28xx/em28xx-i2c.c:31:
drivers/media/video/tuner-xc2028.h:10:26: error: dvb_frontend.h: No such file or directory

In file included from drivers/media/video/em28xx/em28xx-i2c.c:31:
drivers/media/video/tuner-xc2028.h:32: warning: 'struct dvb_frontend' declared inside parameter list
drivers/media/video/tuner-xc2028.h:32: warning: its scope is only this definition or declaration, which is probably not what you want

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