net: Move prototype declaration to include/net/ipx.h from net/ipx/ipx_route.c

Move prototype definition of function to header file include/net/ipx.h
from net/ipx/ipx_route.c because they are used by more than one file.

This eliminates the following warning from net/ipx/af_ipx.c:
net/ipx/af_ipx.c:193:23: warning: no previous prototype for ‘ipxitf_find_using_net’ [-Wmissing-prototypes]
net/ipx/af_ipx.c:577:5: warning: no previous prototype for ‘ipxitf_send’ [-Wmissing-prototypes]
net/ipx/af_ipx.c:1219:8: warning: no previous prototype for ‘ipx_cksum’ [-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>