libctf: link: remember to turn off the LCTF_LINKING flag after ctf_link_write

We set this flag at the top of ctf_link_write (to tell ctf_serialize, way
down under the archive file writing functions, to do the various link- time
serialization things like symbol filtering and the like), but we never
remember to clear it except on error.  This is probably bad if you want to
serialize the dict yourself directly in the future after linking it (which
is...  definitely a *possible* use of the API, if rather strange).

libctf/
	* ctf-link.c (ctf_link_write): Clear LCTF_LINKING before exit.