fix warning (#7174)

Author: NikVolf

primitives/core/src/sr25519.rs

@@ -27,8 +27,8 @@ use sp_std::vec::Vec;
 use schnorrkel::{signing_context, ExpansionMode, Keypair, SecretKey, MiniSecretKey, PublicKey,
 	derive::{Derivation, ChainCode, CHAIN_CODE_LENGTH}
 };
-#[cfg(feature = "full_crypto")]
-use core::convert::TryFrom;
+#[cfg(feature = "std")]
+use std::convert::TryFrom;
 #[cfg(feature = "std")]
 use substrate_bip39::mini_secret_from_entropy;
 #[cfg(feature = "std")]