[media] pwc-if: get rid of warning: no previous prototype

drivers/media/usb/pwc/pwc-if.c:158:23: warning: no previous prototype for 'pwc_get_next_fill_buf' [-Wmissing-prototypes]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/pwc/pwc-if.c

@@ -155,7 +155,7 @@ static struct video_device pwc_template = {
 /***************************************************************************/
 /* Private functions */
 
-struct pwc_frame_buf *pwc_get_next_fill_buf(struct pwc_device *pdev)
+static struct pwc_frame_buf *pwc_get_next_fill_buf(struct pwc_device *pdev)
 {
 	unsigned long flags = 0;
 	struct pwc_frame_buf *buf = NULL;