Add sensor_msgs, geometry_msgs as ament dependencies when BUILD_TESTING is true

[amarburg]

I was trying to build Jazzy from source (long story). While colcon-building foxglove_sdk branch master, I got these cryptic errors:

CMake Error at CMakeLists.txt:124 (add_library):
  Target "foxglove_bridge_component" links to target
  "sensor_msgs::sensor_msgs__rosidl_generator_c" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?


CMake Error at CMakeLists.txt:124 (add_library):
  Target "foxglove_bridge_component" links to target
  "sensor_msgs::sensor_msgs__rosidl_typesupport_fastrtps_c" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?


CMake Error at CMakeLists.txt:124 (add_library):
  Target "foxglove_bridge_component" links to target
  "sensor_msgs::sensor_msgs__rosidl_generator_cpp" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?


CMake Error at CMakeLists.txt:124 (add_library):
  Target "foxglove_bridge_component" links to target
  "sensor_msgs::sensor_msgs__rosidl_typesupport_fastrtps_cpp" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?

(etc)

I believe this is because sensor_msgs and geometry_msgs are not ament dependencies when BUILD_TESTING is true. This patch adds to ament_export_dependencies when appropriate.

I'm not sure how widespread this error is occuring (which ROS distros, etc), or if it's related to recent changes in foxglove-sdk