tty/n_gsm: fix sparse warning

this patch fixes following sparse warning:

n_gsm.c:2827:22: warning: symbol 'tty_ldisc_packet' was not declared. Should it be static?
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2824,7 +2824,7 @@ static int gsm_create_network(struct gsm_dlci *dlci, struct gsm_netconfig *nc)
 }
 
 /* Line discipline for real tty */
-struct tty_ldisc_ops tty_ldisc_packet = {
+static struct tty_ldisc_ops tty_ldisc_packet = {
 	.owner		 = THIS_MODULE,
 	.magic           = TTY_LDISC_MAGIC,
 	.name            = "n_gsm",