Per POSIX 2008, ssize_t needs to exist whether or not we are
replacing functions that use it. For now, we only guarantee
it if we know that the user expects a function that uses
that type, while waiting for glibc to catch up to POSIX.
* lib/unistd.in.h (includes): Ensure that types required by POSIX
2008 are exposed when needed.
* lib/stdio.in.h (includes): Likewise.
Reported by Ralf Wildenhues.