libctf, open: new API for getting the size of CTF/BTF file sections

I wrote this for BTF type size querying programs, but it might be
of more general use and it's impossible to get this info in any
other way, so we might want to keep it.

New API:
+size_t ctf_sect_size (ctf_dict_t *, ctf_sect_names_t sect);

include/ctf-api.h

@@ -894,6 +894,10 @@ extern char *ctf_dump (ctf_dict_t *, ctf_dump_state_t **state,
 		       ctf_sect_names_t sect, ctf_dump_decorate_f *,
 		       void *arg);
 
+/* Return the size in bytes of a given CTF section, or 0 if none.  */
+
+extern size_t ctf_sect_size (ctf_dict_t *, ctf_sect_names_t sect);
+
 /* Error-warning reporting: an 'iterator' that returns errors and warnings from
    the error/warning list, in order of emission.  Errors and warnings are popped
    after return: the caller must free the returned error-text pointer.  */