Fixed duplicate search path issue

[fairlight1337]

Duplicate search paths would generate faulty .asd files form action definitions (request/feedback/reply messages get defined multiple times, i.e. breaking ASDF loading).

Fixes the problem stated on ros-users in this mail thread:
https://code.ros.org/lurker/thread/20130605.125757.7e5ce9d5.en.html

[dirk-thomas]

The patch only addresses the symptoms but not the actual problem of having duplicates in the search path for messages. After looking into it a bit more the issue is not limited to actions but also affects messages if add_message_files() is invoked multiple time in one project. Actions always use multiple calls to add_message_files() which makes it most visible there. I have created a ticket on genmsg for that where the actual issue should be addressed: ros/genmsg#27