tree: Use <stdio.h> for snprintf

<stdio.h> header is used for input/output operations (such as printf,
scanf, fopen, etc.). Although some input/output functions can manipulate
strings, they do not need to directly include <string.h> because they
are declared independently.

Change-Id: Ibe2a4ff6f68843a6d99cfdfe182cf2dd922802aa
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82665
Reviewed-by: Yidi Lin <yidilin@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/kontron/986lcd-m/mainboard.c

@@ -6,6 +6,7 @@
 #include <console/console.h>
 #include <drivers/intel/gma/int15.h>
 #include <option.h>
+#include <stdio.h>
 #include <superio/hwm5_conf.h>
 #include <superio/nuvoton/common/hwm.h>