src/c-types-impl.lisp: Fix arg list in `c-function-type' instance init.

Previously, `make-function-type' was responsible for spotting `(void)'
argument lists and converting them into empty lists, so if you used
`make-instance' directly you could sneak an actual `(void)' argument
list int.  Now the instance initialization machinery for the
`c-function-type' class does this itself, so the gap is closed.