Use explicit conversion from PKHash -> CKeyID

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

Note a function is used rather than a casting
operator as CKeyID is defined at a lower level than script/standard

src/script/standard.h

@@ -79,6 +79,7 @@ struct PKHash : public uint160
     explicit PKHash(const uint160& hash) : uint160(hash) {}
     explicit PKHash(const CPubKey& pubkey);
 };
+CKeyID ToKeyID(const PKHash& key_hash);
 
 struct WitnessV0KeyHash;
 struct ScriptHash : public uint160