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

Change-Id: I48ff8175a1e9f6f012e5bed4f7268c5e9b0745e0
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/mboxd_windows.h b/mboxd_windows.h
index b81007d..3625521 100644
--- a/mboxd_windows.h
+++ b/mboxd_windows.h
@@ -21,6 +21,8 @@
 #define NO_FLUSH	false
 #define WITH_FLUSH	true
 
+#include "mbox.h"
+
 /* Initialisation Functions */
 int init_windows(struct mbox_context *context);
 void free_windows(struct mbox_context *context);