export image_transport dependency

[iuhilnehc-ynos]

When I build the nav2_rviz_plugins package which depends on rviz_default_plugins, it failed as follows,

--- stderr: nav2_rviz_plugins                                                                                                       
CMake Error at CMakeLists.txt:50 (add_library):
  Target "nav2_rviz_plugins" links to target
  "image_transport::image_transport" but the target was not found.  Perhaps a
  find_package() call is missing for an IMPORTED target, or an ALIAS target
  is missing?

I think it's better to use ament_export_dependencies for the image_transport rather than add the find_package(image_transport) in nav2_rviz_plugins.

Signed-off-by: Chen Lihui lihui.chen@sony.com

[clalancette]

Looks good with green CI.

[iuhilnehc-ynos]

CI:
Build args: --packages-above-and-dependencies rviz_default_plugins
Test args: --packages-above rviz_default_plugins

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