mboxd_flash: Include mbox.h to make header self-contained

Change-Id: Ia8b1c514c2a102db462b376e5dc90c656655420c
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/mboxd_flash.h b/mboxd_flash.h
index 69bc121..af9757b 100644
--- a/mboxd_flash.h
+++ b/mboxd_flash.h
@@ -21,6 +21,8 @@
 #define FLASH_DIRTY	0x00
 #define FLASH_ERASED	0x01
 
+#include "mbox.h"
+
 int init_flash_dev(struct mbox_context *context);
 void free_flash_dev(struct mbox_context *context);
 int copy_flash(struct mbox_context *context, uint32_t offset, void *mem,