libpayload: Add ptrdiff_t typedef

vboot2 in payloads (eg depthcharge) needs it.

Change-Id: I4e79ae29cc282c8680f21686befd35c4ff461b3a
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/11922
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

payloads/libpayload/include/stdint.h

@@ -38,6 +38,7 @@ typedef unsigned long long uintmax_t;
 typedef unsigned long uintptr_t;
 typedef long intptr_t;
 
+typedef long ptrdiff_t;
 
 #define INT8_MAX	(127)
 #define INT16_MAX	(32767)