src/builtin.lisp (class-slot "init"): Fix declaration of temporaries.

src/builtin.lisp (class-slot "init"): Fix declaration of temporaries.

Each slot initializer is first used to initialize a temporary of the
correct type, which is then assigned to the slot's structure
member.  (This was introduced in commit 1d8206e.)  Unfortunately, the
temporary was declared naïvely, as `TYPE NAME', which works when TYPE
involves only declaration specifiers and pointer declarator operators,
but not otherwise.

Fix this, by using the `pprint-c-type' function like it was meant to be.