moveit_ros/perception: address gcc6 build error

Since gcc6, compiling fails with `stdlib.h: No such file or directory`,
as including '-isystem /usr/include' breaks since gcc6, cf.,
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129.

This commit addresses this issue for this package in the same way
it was addressed in various other ROS packages. A list of related
commits and pull requests is at:

  ros/rosdistro#12783

Commit 4417f1e ("making some includes SYSTEM and re-adding link_directories")
on 2012-12-06 added the SYSTEM without providing any further
rationale for it.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>

moveit_ros/perception/CMakeLists.txt

@@ -84,8 +84,6 @@ include_directories(lazy_free_space_updater/include
                     ${perception_GL_INCLUDE_DIRS}
                     ${Boost_INCLUDE_DIRS}
                     ${catkin_INCLUDE_DIRS}
-                    )
-include_directories(SYSTEM
                     ${OpenCV_INCLUDE_DIRS}
                     ${EIGEN3_INCLUDE_DIRS}
                     ${SYSTEM_GL_INCLUDE_DIR}