runtime: staking-handler: Add no_std module marker.

runtime-modules/staking-handler/src/lib.rs

@@ -1,3 +1,6 @@
+// Ensure we're `no_std` when compiling for Wasm.
+#![cfg_attr(not(feature = "std"), no_std)]
+
 use frame_support::dispatch::{DispatchError, DispatchResult};
 use frame_support::traits::{Currency, Get, LockIdentifier, LockableCurrency, WithdrawReasons};
 use sp_arithmetic::traits::Zero;