net: Move prototype declaration to header file include/net/dn.h from net/decnet/af_decnet.c

Move prototype declaration of functions to header file include/net/dn.h
from net/decnet/af_decnet.c because they are used by more than one file.

This eliminates the following warning in net/decnet/af_decnet.c:
net/decnet/sysctl_net_decnet.c:354:6: warning: no previous prototype for ‘dn_register_sysctl’ [-Wmissing-prototypes]
net/decnet/sysctl_net_decnet.c:359:6: warning: no previous prototype for ‘dn_unregister_sysctl’ [-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>