sandbox: Always enable malloc debug

Tun on DEBUG in malloc(). This adds code space and slows things down but
for sandbox this is acceptable. We gain the ability to check for memory
leaks in tests.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/dlmalloc.c

@@ -1,5 +1,9 @@
 #include <common.h>
 
+#ifdef CONFIG_SANDBOX
+#define DEBUG
+#endif
+
 #if 0	/* Moved to malloc.h */
 /* ---------- To make a malloc.h, start cutting here ------------ */