Fix libvoxel_grid.so conflict

[drjsmith]

After following the installation instructions, I was able to run the provided examples just fine. However, I discovered that my Turtlebot-based navigation configurations no longer functioned properly:

.../system_catkin_ws/rinstall/lib/move_base/move_base: symbol lookup error: 
.../system_catkin_ws/rinstall/lib/liblayers.so: undefined symbol: _ZN10voxel_grid9VoxelGridC1Ejjj

Long story short, the source of the problem was that the libvoxel_grid.so library created by planning_ros_utils/CMakeLists.txt had the same name as the library produced by the voxel_grid package in the ROS Navigation Stack.

Changing the name of the library to something unique resolves the issue.