samples: keys: random_hw_unique_key: Fix sample.yml

`nrf21540dk_nrf52840` was present in integration platforms but not in
the platform allowlist, causing CI failue.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

samples/keys/random_hw_unique_key/sample.yaml

@@ -3,7 +3,7 @@ sample:
   name: Random HW Unique Key
 common:
     tags: hw_unique_key
-    platform_allow: nrf5340dk_nrf5340_cpuapp nrf9160dk_nrf9160 nrf52840dk_nrf52840
+    platform_allow: nrf5340dk_nrf5340_cpuapp nrf9160dk_nrf9160 nrf52840dk_nrf52840 nrf21540dk_nrf52840
     integration_platforms:
       - nrf5340dk_nrf5340_cpuapp
       - nrf9160dk_nrf9160