netfilter: fix CONFIG_NF_REJECT_IPV6=m link error

We get a new link error with CONFIG_NFT_REJECT_INET=y and CONFIG_NF_REJECT_IPV6=m
after larger parts of the nftables modules are linked together:

net/netfilter/nft_reject_inet.o: In function `nft_reject_inet_eval':
nft_reject_inet.c:(.text+0x17c): undefined reference to `nf_send_unreach6'
nft_reject_inet.c:(.text+0x190): undefined reference to `nf_send_reset6'

The problem is that with NF_TABLES_INET set, we implicitly try to use
the ipv6 version as well for NFT_REJECT, but when CONFIG_IPV6 is set to
a loadable module, it's impossible to reach that.

The best workaround I found is to express the above as a Kconfig
dependency, forcing NFT_REJECT itself to be 'm' in that particular
configuration.

Fixes: 02c7b25e ("netfilter: nf_tables: build-in filter chain type")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>