[media] sh_vou: declare static functions as such

drivers/media/platform/sh_vou.c:799:5: warning: no previous prototype for 'sh_vou_g_output' [-Wmissing-prototypes]
 int sh_vou_g_output(struct file *file, void *fh, unsigned int *i)
     ^
drivers/media/platform/sh_vou.c:805:5: warning: no previous prototype for 'sh_vou_s_output' [-Wmissing-prototypes]
 int sh_vou_s_output(struct file *file, void *fh, unsigned int i)
     ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>