blktap: add one static.

one static.

Signed-off-by: Isaku Yamahata <yamahata@xxxxxxxxxxxxx>

diff --git a/drivers/xen/blktap/blktap.c b/drivers/xen/blktap/blktap.c
--- a/drivers/xen/blktap/blktap.c
+++ b/drivers/xen/blktap/blktap.c
@@ -855,7 +855,7 @@ static unsigned int blktap_poll(struct f
 	return 0;
 }
 
-void blktap_kick_user(int idx)
+static void blktap_kick_user(int idx)
 {
 	tap_blkif_t *info;