tty: n_gsm: depends on NET

n_gsm uses skb functions, so it should depend on NET.

n_gsm.c:(.text+0x123d49): undefined reference to `skb_dequeue'
n_gsm.c:(.text+0x123d98): undefined reference to `kfree_skb'
n_gsm.c:(.text+0x123e1e): undefined reference to `skb_pull'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>