parser_bison: keep snat/dnat existing syntax unchanged

parser_bison: keep snat/dnat existing syntax unchanged

We should keep existing syntax unchanged, and this was emphasized
in the commit 850f0a56b6ad ("src: add 'to' for snat and dnat")'s
commit log: "Existing syntax is still preserved, but the listing
shows the one including 'to'."

This problem was found by running shell test:
  # ./run-tests.sh
  [ ... ]
  W: [FAILED]	./testcases/maps/anonymous_snat_map_0
  I: [OK]		./testcases/maps/map_with_flags_0
  W: [FAILED]	./testcases/maps/named_snat_map_0
  [ ... ]

Fixes: 850f0a56b6ad ("src: add 'to' for snat and dnat")
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>