libctf: work around an uninitialized variable warning

GCC 11+ complains that sym is uninitialized in ctf_symbol_next.  It
isn't, but it's not quite smart enough to figure that out (it requires
domain-specific knowledge of the state of the ctf_next_t iterator
over multiple calls).

libctf/
	* ctf-lookup.c (ctf_symbol_next): Initialize sym to a suitable
	value for returning if never reset during the function.

libctf/ctf-lookup.c

@@ -651,7 +651,7 @@ ctf_id_t
 ctf_symbol_next (ctf_dict_t *fp, ctf_next_t **it, const char **name,
 		 int functions)
 {
-  ctf_id_t sym;
+  ctf_id_t sym = CTF_ERR;
   ctf_next_t *i = *it;
   int err;