[NETFILTER]: Make sure xt_policy.h is unifdef'ed.

Since the header file xt_policy.h tests __KERNEL__, it should be
unifdef'ed before exporting to userspace.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>