libctf, testsuite: adjust for real return type of ctf_member_count

This returns an int, not a long int or an ssize_t (as one test was
inconsistently assuming).

libctf/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* testsuite/libctf-lookup/struct-iteration.c (main):
	ctf_member_count returns an int.