[franka_hw] Install plugin xml to enable to load FrankaCombinableHW from apt version of franka_hw

[pazeshun]

When I try to load franka_hw/FrankaCombinableHW from apt version of franka_hw, I faced the following error:

[ERROR] [/dual_panda] [1668147868.535937528]: Skipped loading plugin with error: XML Document '/opt/ros/melodic/share/franka_hw/franka_combinable_hw_plugin.xml' has no Root Element. This likely means the XML is malformed or missing..
[ERROR] [/dual_panda] [1668147868.537817487]: Could not load robot HW 'rarm' because robot HW type 'franka_hw/FrankaCombinableHW' does not exist.
[ERROR] [/dual_panda] [1668147868.555089521]: franka_combined_control_node:: Initialization of FrankaCombinedHW failed!

This is because franka_hw/franka_combinable_hw_plugin.xml is not installed:
https://github.com/frankaemika/franka_ros/blob/a58d3052a241304392847df6464234c83d728a38/franka_hw/CMakeLists.txt#L104-L112

This PR installs franka_hw/franka_combinable_hw_plugin.xml.
I confirmed this PR works by:

catkin init
catkin config --install
catkin build
source <WORKSPACE_PATH>/install/setup.bash

cf. https://answers.ros.org/question/274849/gmapping-skipping-xml-document/

[rickstaa]

@pazeshun, thanks for your pull request. It fixes a trivial issue 👍🏻. I tested it using the franka_combined_control.launch file, and it works. I hope @frankaemika will merge it soon 🎉.