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>