src/include: Remove unused <stdlib.h>

Change-Id: I9e5d18739e7c5b5c742a905ac482529c7e0866df
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37827
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/string.h

@@ -2,7 +2,6 @@
 #define STRING_H
 
 #include <stddef.h>
-#include <stdlib.h>
 
 #include <console/vtxprintf.h>