[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:

franka_ros/franka_hw/CMakeLists.txt

Lines 104 to 112 in a58d305

	## Installation
	install(TARGETS franka_hw franka_control_services
	ARCHIVE DESTINATION ${CATKIN_PACKAGE_LIB_DESTINATION}
	LIBRARY DESTINATION ${CATKIN_PACKAGE_LIB_DESTINATION}
	RUNTIME DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
	)
	install(DIRECTORY include/${PROJECT_NAME}/
	DESTINATION ${CATKIN_PACKAGE_INCLUDE_DESTINATION}
	)

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 🎉.