sdbus++: events: add object_path include

Event metadata often includes object_path.  Add the header file
into the generated events.hpp files.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia3da51483d25c6f65e6bcf977d47f65d59a37748
diff --git a/tools/sdbusplus/templates/events.hpp.mako b/tools/sdbusplus/templates/events.hpp.mako
index bb9eb85..33d7824 100644
--- a/tools/sdbusplus/templates/events.hpp.mako
+++ b/tools/sdbusplus/templates/events.hpp.mako
@@ -6,6 +6,7 @@
 #include <unistd.h>
 
 #include <sdbusplus/exception.hpp>
+#include <sdbusplus/message.hpp>
 
 #include <cerrno>
 #include <source_location>