phonet: fix build when PROC_FS is disabled

Fix phonet build when PROC_FS is not enabled:

net/built-in.o: In function `pn_sock_open':
socket.c:(.text+0x23c649): undefined reference to `seq_open_net'
net/built-in.o:(.rodata+0x21018): undefined reference to `seq_release_net'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>