libctf: fix a comment

A mistaken "not" in ctf_err_warn made it seem like we only extracted
error messages if this was not an error.

libctf/

	* ctf-subr.c (ctf_err_warn): Fix comment.

libctf/ctf-subr.c

@@ -225,7 +225,7 @@ ctf_err_warn (ctf_dict_t *fp, int is_warning, int err,
     }
   va_end (alist);
 
-  /* Include the error code only if there is one; if this is not a warning,
+  /* Include the error code only if there is one; if this is a warning,
      only use the error code if it was explicitly passed and is nonzero.
      (Warnings may not have a meaningful error code, since the warning may not
      lead to unwinding up to the user.)  */