test: Support testing malloc() failures

It is helpful to test that out-of-memory checks work correctly in code
that calls malloc().

Add a simple way to force failure after a given number of malloc() calls.

Fix a header guard to avoid a build error on sandbox_vpl.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
diff --git a/arch/sandbox/include/asm/malloc.h b/arch/sandbox/include/asm/malloc.h
index a1467b5..8aaaa9c 100644
--- a/arch/sandbox/include/asm/malloc.h
+++ b/arch/sandbox/include/asm/malloc.h
@@ -6,6 +6,7 @@
  */
 
 #ifndef __ASM_MALLOC_H
+#define __ASM_MALLOC_H
 
 void *malloc(size_t size);
 void free(void *ptr);