src: Add missing include <stdint.h>

Change-Id: Idf10a09745756887a517da4c26db7a90a1bf9543
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/29403
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/arch/x86/include/arch/registers.h

@@ -17,6 +17,8 @@
 #define __ARCH_REGISTERS_H
 
 #if !defined(__ASSEMBLER__)
+#include <stdint.h>
+
 #define DOWNTO8(A) \
 	union { \
 		struct { \