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);

libctf/libctf.ver

@@ -167,6 +167,7 @@ LIBCTF_2.0 {
 	ctf_write_mem;
 	ctf_gzwrite;
 	ctf_compress_write;
+	ctf_sect_size;
 	ctf_getdatasect;
 	ctf_getsymsect;
 	ctf_getstrsect;