netfilter: select NFNETLINK when enabling NF_TABLES

In Kconfig, nf_tables depends on NFNETLINK so building nf_tables as
a module or inside kernel depends on the state of NFNETLINK inside
the kernel config. If someone wants to build nf_tables inside the
kernel, it is necessary to also build NFNETLINK inside the kernel.
But NFNETLINK can not be set in the menu so it is necessary to
toggle other nfnetlink subsystems such as logging and nfacct to see
the nf_tables switch.

This patch changes the dependency from 'depend' to 'select' inside
Kconfig to allow to set the build of nftables as modules or inside
kernel independently.

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>