[CMake] set RPATH for plugins

to fix debian packages creation on ROS buildfarm:

00:12:11 dpkg-shlibdeps: error: cannot find library libtracer.so.4.3.1 needed by
debian/ros-noetic-dynamic-graph/opt/ros/noetic/lib/dynamic-graph-plugins/libtracer-real-time.so.4.3.1
(ELF format: 'elf64-x86-64' abi: '0201003e00000000'; RPATH: '')