libc: Add definition for ssize_t

Add definition for ssize_t that may be useful to represent sizes with
variables that also need to be able to have negative values (e.g. to
represent errors).

Change-Id: I52ec69591ccfd760021dad38f5ba1b1ebe707ea5
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
2 files changed