libctf: don't leak the symbol name in the name->type cache

This cache replaced a cache of symbol index->ctf_id_t. That cache was
just an array, so it could get away with just being free()d, but the
ctfi_symnamedicts cache that replaced it is a full dynhash with a
dynamically-allocated string as the key.  As such, it needs freeing with
ctf_dynhash_destroy(), not just free(), or we leak parts of the
underlying hashtab, and all the keys.

libctf/ChangeLog:

	* ctf-archive.c (ctf_arc_flush_caches): Fix leak.