Fix warnings about calling conventions

[hoffmann-stefan]

extracted from #94

This fixes all colcon build warnings for foxi on linux.
There are some new ones for humbe, but they will be fixed seperatly.

In file included from /home/stefan/public_git/ros2_dotnet_ws/build/unique_identifier_msgs/rosidl_generator_dotnet/unique_identifier_msgs/msg/uuid.c:20:
/home/stefan/public_git/ros2_dotnet_ws/build/unique_identifier_msgs/rosidl_generator_dotnet/unique_identifier_msgs/msg/rcldotnet_uuid.h:27:1: warning: ‘cdecl’ attribute ignored [-Wattributes]
   27 | const void * RCLDOTNET_UNIQUE_IDENTIFIER_MSGS_UNIQUE_IDENTIFIER_MSGS_MSG_UUID_CDECL unique_identifier_msgs__msg__UUID__get_typesupport();
      | ^~~~~

From the commit message:

This removes a warning when compiling on x64 plattforms.
The C# DllImportAttribute ignores this calling canvention as well, but dosn't give a warning.