lib/bootsplash: Fix compilation

Add missing include.
Tested on Supermicro X11SSH-TF.

Change-Id: Id0c80ac646001a497e96cae4d48a0f09762eb936
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35613
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/bootsplash.c

@@ -18,6 +18,7 @@
 #include <console/console.h>
 #include <endian.h>
 #include <bootsplash.h>
+#include <stdlib.h>
 
 #include "jpeg.h"