samples: bluetooth: central_uart: validate nRF54LM20 Dongle build on PR

Until nRF54ML20 Dongle has enough CI coverage, at least validate on PR
that central_uart builds for the dongle.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>

Author: Damian-Nordic

samples/bluetooth/central_uart/sample.yaml

@@ -12,6 +12,7 @@ tests:
     sysbuild: true
     integration_platforms:
       - nrf54l15dk/nrf54l15/cpuapp
+      - nrf54lm20dongle/nrf54lm20b/cpuapp
     platform_allow:
       - nrf52dk/nrf52832
       - nrf52840dk/nrf52840