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>