ctfout: use ctfc_get_num_ctf_vars instead

A minor cosmetic fix.

2022-03-28  Indu Bhagat  <indu.bhagat@oracle.com>

gcc/ChangeLog:

	* ctfout.cc (ctf_preprocess): Use ctfc_get_num_ctf_vars instead.
	(output_ctf_vars): Likewise.