tty: Fix sparse static warning for tty_driver_lookup_tty

Fixed sparse warning:
drivers/char/tty_io.c:1216:19: warning: symbol 'tty_driver_lookup_tty' was not declared. Should it be static?

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index d8d240c..2a15af6 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1211,7 +1211,7 @@
  *	be held until the 'fast-open' is also done. Will change once we
  *	have refcounting in the driver and per driver locking
  */
-struct tty_struct *tty_driver_lookup_tty(struct tty_driver *driver,
+static struct tty_struct *tty_driver_lookup_tty(struct tty_driver *driver,
 		struct inode *inode, int idx)
 {
 	struct tty_struct *tty;