[media] cx25821-video: declare cx25821_vidioc_s_std as static

Fixes the following warning:

	drivers/media/pci/cx25821/cx25821-video.c: At top level:
	drivers/media/pci/cx25821/cx25821-video.c:766:5: warning: no previous prototype for 'cx25821_vidioc_s_std' [-Wmissing-prototypes]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/pci/cx25821/cx25821-video.c

@@ -762,7 +762,8 @@ static int cx25821_vidioc_g_std(struct file *file, void *priv, v4l2_std_id *tvno
 	return 0;
 }
 
-int cx25821_vidioc_s_std(struct file *file, void *priv, v4l2_std_id tvnorms)
+static int cx25821_vidioc_s_std(struct file *file, void *priv,
+				v4l2_std_id tvnorms)
 {
 	struct cx25821_channel *chan = video_drvdata(file);
 	struct cx25821_dev *dev = chan->dev;