src/include: Add missing includes

Change-Id: I746ea7805bae553a146130994d8174aa2e189610
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43368
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/cpu/intel/l2_cache.h

@@ -13,6 +13,8 @@
 #ifndef __P6_L2_CACHE_H
 #define __P6_L2_CACHE_H
 
+#include <stdint.h>
+
 #define EBL_CR_POWERON	0x2A
 
 #define BBL_CR_D0	0x88