net: Move prototype declaration to header file include/net/ipx.h from net/ipx/af_ipx.c

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

This eliminates the following warning in
net/ipx/ipx_route.c:33:19: warning: no previous prototype for ‘ipxrtr_lookup’ [-Wmissing-prototypes]
net/ipx/ipx_route.c:52:5: warning: no previous prototype for ‘ipxrtr_add_route’ [-Wmissing-prototypes]
net/ipx/ipx_route.c:94:6: warning: no previous prototype for ‘ipxrtr_del_routes’ [-Wmissing-prototypes]
net/ipx/ipx_route.c:149:5: warning: no previous prototype for ‘ipxrtr_route_skb’ [-Wmissing-prototypes]
net/ipx/ipx_route.c:171:5: warning: no previous prototype for ‘ipxrtr_route_packet’ [-Wmissing-prototypes]
net/ipx/ipx_route.c:261:5: warning: no previous prototype for ‘ipxrtr_ioctl’ [-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>