libpayload: Add qsort()

It's taken from OpenBSD and thus appropriately licensed (and reasonably
tested).

Change-Id: I5767600c9865d39e56c220b52e045f3501875b98
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/88
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/stdlib.h

@@ -142,4 +142,6 @@ void exit(int status) __attribute__ ((noreturn));
 
 /** @} */
 
+void qsort(void *aa, size_t n, size_t es, int (*cmp)(const void *, const void *));
+
 #endif