netfilter: conntrack: gre: switch module to be built-in

This makes the last of the modular l4 trackers 'bool'.

After this, all infrastructure to handle dynamic l4 protocol registration
becomes obsolete and can be removed in followup patches.

Old:
302824 net/netfilter/nf_conntrack.ko
 21504 net/netfilter/nf_conntrack_proto_gre.ko

New:
313728 net/netfilter/nf_conntrack.ko

Old:
   text	   data	    bss	    dec	    hex	filename
   6281	   1732	      4	   8017	   1f51	nf_conntrack_proto_gre.ko
 108356	  20613	    236	 129205	  1f8b5	nf_conntrack.ko
New:
 112095	  21381	    240	 133716	  20a54	nf_conntrack.ko

The size increase is only temporary.

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