Build fail on fedora 28

[MalcolmMielle]

building pcl 1.8 on fedora 28 fails with:

In file included from /home/malcolm/external_sources/pcl/segmentation/include/pcl/segmentation/organized_multi_plane_segmentation.h:48,
                 from /home/malcolm/external_sources/pcl/segmentation/src/organized_multi_plane_segmentation.cpp:42:
/home/malcolm/external_sources/pcl/segmentation/include/pcl/segmentation/plane_coefficient_comparator.h: In member function ‘const std::vector<float>& pcl::PlaneCoefficientComparator<PointT, PointNT>::getPlaneCoeffD() const’:
/home/malcolm/external_sources/pcl/segmentation/include/pcl/segmentation/plane_coefficient_comparator.h:144:31: error: invalid initialization of reference of type ‘const std::vector<float>&’ from expression of type ‘const boost::shared_ptr<std::vector<float> >’
         return (plane_coeff_d_);
                               ^
make[2]: *** [segmentation/CMakeFiles/pcl_segmentation.dir/build.make:167: segmentation/CMakeFiles/pcl_segmentation.dir/src/organized_multi_plane_segmentation.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:1102: segmentation/CMakeFiles/pcl_segmentation.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

• Operating System and version: Fedora 28
• Compiler: gcc8
• PCL Version: 1.8

[taketwo]

Fixed by #2299.