mbox: Add includes to header for used types

mbox.h makes use of struct pollfd and bool types, so lets include the
appropriate headers to make the mbox header self-contained.

Change-Id: Ia9e1ce4477a510fb9a0fa841f6542327930e02bb
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/mbox.h b/mbox.h
index 087975d..be9b953 100644
--- a/mbox.h
+++ b/mbox.h
@@ -20,6 +20,8 @@
 
 #include <mtd/mtd-abi.h>
 #include <systemd/sd-bus.h>
+#include <poll.h>
+#include <stdbool.h>
 
 enum api_version {
 	API_VERSION_INVAL	= 0,