netfilter: xt_rpfilter: depend on raw or mangle table

rpfilter is only valid in raw/mangle PREROUTING, i.e.
RPFILTER=y|m is useless without raw or mangle table support.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>