Add missing include file

Change-Id: I2816127a997646fe1ff9ea2601beb84a5b37280e
diff --git a/drivers/mailbox/include/mailbox.hpp b/drivers/mailbox/include/mailbox.hpp
index 9074754..9db3499 100644
--- a/drivers/mailbox/include/mailbox.hpp
+++ b/drivers/mailbox/include/mailbox.hpp
@@ -19,6 +19,7 @@
 #ifndef MAILBOX_HPP
 #define MAILBOX_HPP
 
+#include <cstdint>
 #include <cstddef>
 #include <list>