include/uapi/linux/ipx.h: fix conflicting defitions with glibc netipx/ipx.h

Fixes these compiler warnings via libc-compat.h when glibc netipx/ipx.h is
included before linux/ipx.h:

./linux/ipx.h:9:8: error: redefinition of ‘struct sockaddr_ipx’
./linux/ipx.h:26:8: error: redefinition of ‘struct ipx_route_definition’
./linux/ipx.h:32:8: error: redefinition of ‘struct ipx_interface_definition’
./linux/ipx.h:49:8: error: redefinition of ‘struct ipx_config_data’
./linux/ipx.h:58:8: error: redefinition of ‘struct ipx_route_def’

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>