Fix node-rpc-client compilation (#4159)

* Fix `node-rpc-client` compilation

* Enable `alloc` feature of `zeroize` on `std`

substrate/primitives/timestamp/src/lib.rs

@@ -23,7 +23,7 @@ use codec::Encode;
 use codec::Decode;
 #[cfg(feature = "std")]
 use inherents::ProvideInherentData;
-use inherents::{InherentIdentifier, ProvideInherent, IsFatalError, InherentData};
+use inherents::{InherentIdentifier, IsFatalError, InherentData};
 
 use sr_primitives::RuntimeString;
 
@@ -112,4 +112,4 @@ impl ProvideInherentData for InherentDataProvider {
 #[impl_trait_for_tuples::impl_for_tuples(30)]
 pub trait OnTimestampSet<Moment> {
 	fn on_timestamp_set(moment: Moment);
-}
+}