exports: Add the malloc.h header

This file should include the malloc.h header since it references malloc().
Fix it.

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

common/exports.c

 #include <common.h>
 #include <exports.h>
+#include <malloc.h>
 #include <spi.h>
 #include <i2c.h>