Fixes #5113: Correct rudder-agent-thin Makefile to have a correct localc...

...lean target

Ticket: http://www.rudder-project.org/redmine/issues/5113

To be merged in 2.11
I'm pretty sure you should list ../../rudder-agent/SOURCES/Makefile as a dependency here. This will enable Make to "know" when this needs copying again.