mboxd_msg: Include mbox.h to avoid undefined symbol error

mboxd_msg.h uses the MBOX_REG_BYTES symbol, but forces its users to
first include mbox.h. Include what we use to restore some sanity.

Change-Id: Ia72de84e6ba8abc86044170a355a4d43f96a239e
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/mboxd_msg.h b/mboxd_msg.h
index 94ac13a..124927c 100644
--- a/mboxd_msg.h
+++ b/mboxd_msg.h
@@ -5,6 +5,7 @@
 #define MBOXD_MSG_H
 
 #include "common.h"
+#include "mbox.h"
 
 /* Estimate as to how long (milliseconds) it takes to access a MB from flash */
 #define FLASH_ACCESS_MS_PER_MB		8000