add rosidl_core repo to ros2_dotnet_rolling.repos

In ros2/rosidl_defaults#22, the rosidl_core_generators package was split off from rosidl_defaults into a separate repo, which needed to be added.
Without this repository the dotnet code for the interfaces is not generated in the workspace.

see PR on ros2-rust: ros2-rust/ros2_rust#251

ros2_dotnet_rolling.repos

@@ -11,6 +11,10 @@ repositories:
     type: git
     url: https://github.com/ros2/rcl_interfaces.git
     version: rolling
+  ros2/rosidl_core:
+    type: git
+    url: https://github.com/ros2/rosidl_core.git
+    version: rolling
   ros2/rosidl_defaults:
     type: git
     url: https://github.com/ros2/rosidl_defaults.git