fix libc glob reexports change: use c_char instead of c_schar

mozilla/rust/bfaf171c6df removes all the glob reexports from liblibc, making ``c_schar`` missing from libc top level. (now only in ``libc::types::os::arch::c95::c_schar``)

https://github.com/mozilla/rust/commit/bfaf171c6dff38faecf4de29abcedc6a128c4cec

switch to ``c_char`` is ok, because they are the same type. :)