* Remove user_interface.h from gdbstub includes
Avoid a warning by not including the unneeded "user_interface.h" from
gdbstub.c. Leftover from original stub code from the SDK, this include
is unnecessary in the Arduino core.
* Add example to ensure gdbstub compiles in CI
Ensure the same user_interface warning doesn't occur in the future by
adding the example shown in the GDB docs to the CI build process.
