libctf: create: don't add forwards if the type added already exists

libctf: create: don't add forwards if the type added already exists

This is what ctf_add_forward is documented to do, but it's not what it
actually does: the code is quite happy to add forwards that duplicate
existing structs, etc.

This is obviously wrong and breaks both the nondeduplicating linker
and the upcoming deduplicator, as well as allowing ordinary callers of
ctf_add_type to corrupt the dictionary by just adding the same root-
visible forward more than once.

libctf/
	* ctf-create.c (ctf_add_forward): Don't add forwards to
	types that already exist.