samples: sensor: accel_trig: add arduino_gpio req + platform_allow entry

sample.sensor.accel_trig.shield-tap has a DT overlay that expects the
presence of an arduino_header, so add the appropriate dependency to only
build the sample against supported boards.

As per samples guidelines, use platform_allow to limit CI to run only on
a configuration that's actually been tested (and is documented in the
README)

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>