parser: allow typeof in objref maps

parser: allow typeof in objref maps

Its currently not possible to declare a map that
stores object references with the "typeof" keyword, e.g.

map m {
	type ipv4_addr : limit

will work, but

map m {
	typeof ip saddr : limit

will give a syntax error ("unexpected limit").
Followup pach will add support for listing side too.

Signed-off-by: Florian Westphal <fw@strlen.de>