[fix] export cmake library install

[tylerjw]

This fixes an issue with the cmake install that causes problems when using this in an underlay workspace.

When you build this in an underlay and then try to build the overlay repo where something depends on it you get an error like this:

--- stderr: moveit_core                          
moveit_core: You did not request a specific build type: Choosing 'Release' for maximum performance
CMake Error at /opt/underlay_ws/install/srdfdom/share/srdfdom/cmake/ament_cmake_export_libraries-extras.cmake:48 (message):
  Package 'srdfdom' exports the library 'srdfdom' which couldn't be found

Documentation reference: https://index.ros.org/doc/ros2/Tutorials/Ament-CMake-Documentation/

[rhaschke]

According to the cmake doc:

The EXPORT option associates the installed target files with an export called . It must appear before any RUNTIME, LIBRARY, or ARCHIVE options. To actually install the export file itself, call install(EXPORT), documented below.

you are missing the call to install(EXPORT), don't you? Or is this installed by ament_package()?

[tylerjw]

I think it is done by ament_package, because this change fixes the build error I saw.