gluon-core: build in xt_CT with GLUON_SPECIALIZE_KERNEL

xt_CT was added to kmod-ipt-conntrack a while back. Update our
GLUON_SPECIALIZE_KERNEL rules accordingly.

When building xt_CT into the kernel, one of the symbols IP_NF_RAW and
IP6_NF_RAW must be enabled as well, even though there is no runtime
dependency when building as modules. The kernel grows by less than 1KB
even when both IP_NF_RAW and NETFILTER_XT_TARGET_CT are enabled, making
this change a (very slight) net win in both flash and RAM usage.