Fix path to soem headers

[garaemon]

This change is required In order to compile robotiq_ethercat package with soem source code. soem package provides include path with soem prefix and 'soem/' prefix in include macro is not required.

If ros-kinetic-soem apt package is installed instead of building with source code, wrong include path to soem headers does not cause any error because /opt/ros/kinetic/include is provided by catkin_INCLUDE_DIRS.