libpayload: x86/exec - fix libpayload API magic value

According to coreboot’s payload API [1] the magic value passed to the
payload should be 0x12345678, not 12345678. Fix that.

[1] https://www.coreboot.org/Payload_API

Change-Id: I10a7f7b1a4aec100416c5e7e4ba7f8add10ef5c5
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: https://review.coreboot.org/18331
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

payloads/libpayload/arch/x86/exec.S

@@ -66,7 +66,7 @@ i386_do_exec:
 	 * and argv are sane
 	 */
 
-	movl  $12345678, %ecx
+	movl  $0x12345678, %ecx
 	pushl %ecx
 
 	/* Jump to the code */