net: Move prototype declaration to appropriate header file from decnet/af_decnet.c

Move prototype declaration of functions to header file include/net/dn_route.h
from net/decnet/af_decnet.c because it is used by more than one file.

This eliminates the following warning in net/decnet/dn_route.c:
net/decnet/dn_route.c:629:5: warning: no previous prototype for ‘dn_route_rcv’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/dn_route.h

···
 			 struct sock *sk, int flags);
 int dn_cache_dump(struct sk_buff *skb, struct netlink_callback *cb);
 void dn_rt_cache_flush(int delay);
+int dn_route_rcv(struct sk_buff *skb, struct net_device *dev,
+		 struct packet_type *pt, struct net_device *orig_dev);
 
 /* Masks for flags field */
 #define DN_RT_F_PID 0x07 /* Mask for packet type                      */

net/decnet/af_decnet.c

···
 	.notifier_call = dn_device_event,
 };
 
-extern int dn_route_rcv(struct sk_buff *, struct net_device *, struct packet_type *, struct net_device *);
-
 static struct packet_type dn_dix_packet_type __read_mostly = {
 	.type =		cpu_to_be16(ETH_P_DNA_RT),
 	.func =		dn_route_rcv,