builtins: Fix comment typo mpft_t -> mpfr_t

I've noticed 4 typos in comments, fixed thusly.

2023-05-05  Jakub Jelinek  <jakub@redhat.com>

	* builtins.cc (do_mpfr_ckconv, do_mpc_ckconv): Fix comment typo,
	mpft_t -> mpfr_t.
	* fold-const-call.cc (do_mpfr_ckconv, do_mpc_ckconv): Likewise.