Include <sys/types.h> wherever ssize_t is used

`ssize_t` is a POSIX type (cf. IEEE Std 1003.1).

Change-Id: I5f6f114523f541b3a8d845c6faee2c0b9f753bae
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reported-by: Urja Rannikko <urjaman@gmail.com>
Reviewed-on: https://review.coreboot.org/21015
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-by: Urja Rannikko <urjaman@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/ich_descriptors.h b/ich_descriptors.h
index 574bc5f..4ddc710 100644
--- a/ich_descriptors.h
+++ b/ich_descriptors.h
@@ -22,6 +22,7 @@
 #ifndef __ICH_DESCRIPTORS_H__
 #define __ICH_DESCRIPTORS_H__ 1
 
+#include <sys/types.h>
 #include <stdint.h>
 #include "programmer.h" /* for enum ich_chipset */