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>

dediprog.c

@@ -21,6 +21,7 @@
 
 #include "platform.h"
 
+#include <sys/types.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>