Fix yes-or-no-p with multibyte spaces

Fix yes-or-no-p with multibyte spaces

Problem reported by Thomas Klausner (Bug#73307).
Emacs shouldn’t use ctype.h, as it doesn’t work for multibyte
chars and it doesn’t work with Emacs’s locale model anyway.
* src/fns.c: Include syntax.h, not ctype.h.
(Fyes_or_no_p): Check the character category with SYNTAX, not
with isspace, which assumes the current locale and works only
with single-byte characters.

(cherry picked from commit 43cde03fa5a663a1509a762077c11eb57a60cee8)