[media] cx18: get rid of warning: no previous prototype

drivers/media/pci/cx18/cx18-alsa-main.c:200:5: warning: no previous prototype for 'cx18_alsa_load' [-Wmissing-prototypes]
drivers/media/pci/cx18/cx18-alsa-pcm.c:325:5: warning: no previous prototype for 'snd_cx18_pcm_create' [-Wmissing-prototypes]
drivers/media/pci/cx18/cx18-alsa-pcm.c:72:6: warning: no previous prototype for 'cx18_alsa_announce_pcm_data' [-Wmissing-prototypes]
drivers/media/pci/cx18/cx18-streams.c:100:6: warning: no previous prototype for 'cx18_dma_free' [-Wmissing-prototypes]

Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>