Use explicit conversion from WitnessV0KeyHash -> CKeyID

These types are equivalent, in data etc, so they need only their
data cast across.

src/script/standard.h

@@ -106,6 +106,7 @@ struct WitnessV0KeyHash : public uint160
     explicit WitnessV0KeyHash(const uint160& hash) : uint160(hash) {}
     explicit WitnessV0KeyHash(const CPubKey& pubkey);
 };
+CKeyID ToKeyID(const WitnessV0KeyHash& key_hash);
 
 //! CTxDestination subtype to encode any future Witness version
 struct WitnessUnknown