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

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

This eliminates the following warning in net/ipx/sysctl_net_ipx.c:
net/ipx/sysctl_net_ipx.c:33:6: warning: no previous prototype for ‘ipx_register_sysctl’ [-Wmissing-prototypes]
net/ipx/sysctl_net_ipx.c:38:6: warning: no previous prototype for ‘ipx_unregister_sysctl’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>