Build on Ubuntu 20.04

[btalb]

Much like a number of Kimera dependencies, the ROS package fails to build on Ubuntu 20.04 due to a missing definition of the -std=c++14 flag:

• ToniRV/mesh_rviz_plugins#3
• ethz-asl/voxblox#370

I followed the instructions in the README here to get the same errors. The errors disappeared when adding the following to kimera_semantics_ros/CMakeLists.txt:

add_definitions(-std=c++14)

[Z-Jeff]

Thanks, It really works!