libctf: Use ctf_type_t instead of struct ctf_type

For consistency with other CTF toolchain code. No functional change
intended.

Fixes: 105fd928b0b5 ("libctf: Improve check for duplicate SOU definitions in ctf_add_type()")

(cherry picked from commit 2e4311906d8c8dc7a7c726345268253bca6d4acc)