fix generating duplicate include dirs for multiple add_message_files() invocations

Duplicate search paths generate faulty .asd files from message definitions. It is most visible for actions since they call add_message_files() multiple times which leads to the duplicate include dirs.

The problem was brought up on ros-users in this mail thread:
https://code.ros.org/lurker/thread/20130605.125757.7e5ce9d5.en.html
(request/feedback/reply messages get defined multiple times, i.e. breaking ASDF loading)