tree: Remove unused <stdarg.h>

<stdarg.h> header is used to define macros for handling variable
argument lists in functions like printf. It does not depend on the string
or memory manipulation functions provided by <string.h>.
So let follow conventions and include only the necessary headers in each
header file.

Change-Id: I07ffc65b7feefb8ec4ab8dd268113f9ed8d24685
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82664
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/string.h

@@ -3,7 +3,6 @@
 #ifndef STRING_H
 #define STRING_H
 
-#include <stdarg.h> /* IWYU pragma: export */
 #include <stddef.h>
 #include <stdio.h> /* IWYU pragma: export */