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 <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/dn_route.h b/include/net/dn_route.h
index b409ad6..55df993 100644
--- a/include/net/dn_route.h
+++ b/include/net/dn_route.h
@@ -20,6 +20,8 @@
 			 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                      */