[media] s5p-tv: Include missing platform_device.h header

Without this patch we get the following build error:
drivers/media/platform/s5p-tv/mixer_video.c:
In function ‘find_and_register_subdev’:
drivers/media/platform/s5p-tv/mixer_video.c:42:34: error:
‘platform_bus_type’ undeclared (first use in this function)

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>