Introduce a PhantomData field in GenesisConfig (#1116)

This makes it possible to support `GenesisConfig`s that to do not use
generic parameter.

substrate/srml/balances/src/mock.rs

@@ -104,6 +104,7 @@ impl ExtBuilder {
 			transfer_fee: self.transfer_fee,
 			creation_fee: self.creation_fee,
 			reclaim_rebate: 0,
+			_genesis_phantom_data: Default::default(),
 		}.build_storage().unwrap().0);
 		t.into()
 	}