netfilter: package required kmods for nftables

netfilter: package required kmods for nftables

Package new kmods "nf_tables_set" and "nft_objref" which got introduced
with kernel 4.18 and restrict the old "nft_set_rbtree" and "nft_set_hash"
modules to sub-4.18 versions.

Also reorder the nftables related netfilter.mk entries alphabetically
while touching this code section.

Fixes: FS#2699
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2699#comment7450
Signed-off-by: Jo-Philipp Wich <jo@mein.io>