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>

net/phonet/socket.c

@@ -413,6 +413,7 @@ found:
 }
 EXPORT_SYMBOL(pn_sock_get_port);
 
+#ifdef CONFIG_PROC_FS
 static struct sock *pn_sock_get_idx(struct seq_file *seq, loff_t pos)
 {
 	struct net *net = seq_file_net(seq);
@@ -509,3 +510,4 @@ const struct file_operations pn_sock_seq_fops = {
 	.llseek = seq_lseek,
 	.release = seq_release_net,
 };
+#endif