phonet: Fix build.

As reported by Oliver Hartkopp:

net/phonet/pn_dev.c: In function ‘phonet_init_net’:
net/phonet/pn_dev.c:221: error: implicit declaration of function
‘proc_net_fops_create’
net/phonet/pn_dev.c: In function ‘phonet_exit_net’:
net/phonet/pn_dev.c:242: error: implicit declaration of function ‘proc_net_remove’

Signed-off-by: David S. Miller <davem@davemloft.net>