netfilter: nft_objref: make it builtin

nft_objref is needed to reference named objects, it makes
no sense to disable it.

Before:
   text	   data	    bss	    dec	 filename
  4014	    424	      0	   4438	 nft_objref.o
  4174	   1128	      0	   5302	 nft_objref.ko
359351	  15276	    864	 375491	 nf_tables.ko
After:
  text	   data	    bss	    dec	 filename
  3815	    408	      0	   4223	 nft_objref.o
363161	  15692	    864	 379717	 nf_tables.ko

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