arch/x86/timestamp.c: Add missing <stdint.h>

Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Change-Id: I6870fb9f3d41ef5dc6599e979ce0c890a1e145ab
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68034
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

src/arch/x86/timestamp.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
 #include <cpu/x86/tsc.h>
+#include <stdint.h>
 #include <timestamp.h>
 
 uint64_t timestamp_get(void)