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>

net/phonet/pn_dev.c

@@ -27,6 +27,7 @@
 #include <linux/net.h>
 #include <linux/netdevice.h>
 #include <linux/phonet.h>
+#include <linux/proc_fs.h>
 #include <net/sock.h>
 #include <net/netns/generic.h>
 #include <net/phonet/pn_dev.h>