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

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

This eliminates the following warning in net/ipx/pe2.c:
net/ipx/pe2.c:20:24: warning: no previous prototype for ‘make_EII_client’ [-Wmissing-prototypes]
net/ipx/pe2.c:32:6: warning: no previous prototype for ‘destroy_EII_client’ [-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>