nimble/controller: Enable LL encryption if BLE_LL_ISO_BROADCASTER

To support encrypting/decrypting broadcasts, LL Encryption shall be
enabled by BLE_LL_ISO_BROADCASTER.
diff --git a/nimble/controller/syscfg.yml b/nimble/controller/syscfg.yml
index c55107b..19fae47 100644
--- a/nimble/controller/syscfg.yml
+++ b/nimble/controller/syscfg.yml
@@ -614,6 +614,7 @@
     BLE_LL_SCAN_AUX_SEGMENT_CNT: 8
 
 syscfg.vals.BLE_LL_ISO_BROADCASTER:
+    BLE_LL_CFG_FEAT_LE_ENCRYPTION: 1
     BLE_LL_STACK_SIZE: 180
 
 # Enable vendor event on assert in standalone build to make failed assertions in