Fix no-std build

Author: olegnn

substrate/frame/staking/src/lib.rs

@@ -282,6 +282,8 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![recursion_limit = "256"]
 
+extern crate alloc;
+
 #[cfg(feature = "runtime-benchmarks")]
 pub mod benchmarking;
 #[cfg(any(feature = "runtime-benchmarks", test))]

substrate/frame/staking/src/pallet/mod.rs

@@ -50,7 +50,7 @@ const STAKING_ID: LockIdentifier = *b"staking ";
 
 #[frame_support::pallet]
 pub mod pallet {
-    use std::collections::BTreeSet;
+    use alloc::collections::BTreeSet;
 
     use frame_election_provider_support::ElectionDataProvider;