ct: fix "ct l3proto/protocol" syntax broken

ct: fix "ct l3proto/protocol" syntax broken

"l3proto" and "protocol" are still keywords in our grammer, they are not
STRING, so if the user input the following rule, nft will complain that
the syntax is error:
  # nft add t c ct l3proto ipv4
  <cmdline>:1:12-18: Error: syntax error, unexpected l3proto, expecting
  string or mark or packets or bytes
  add t c ct l3proto ipv4
             ^^^^^^^

Fixes: c992153402c7 ("ct: allow resolving ct keys at run time")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>