types: minor build improvements

We don't need all of message.hpp to be included from types.hpp since
all we need is the variant definition.  Instead, just get
sdbusplus/message/types.hpp, which has the variant definition.

message.hpp picks up <memory> so add that where it is now missing
(events.hpp).

Add a missing cstdint include.

Change-Id: Ic033ceafcd982dba3e39592a4d8f64e5b4e65f2e
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/events.hpp b/events.hpp
index 0cdcfbc..735834a 100644
--- a/events.hpp
+++ b/events.hpp
@@ -2,6 +2,9 @@
 
 #include "types.hpp"
 
+#include <memory>
+#include <vector>
+
 namespace phosphor
 {
 namespace inventory