Introduce a PhantomData field in GenesisConfig (#1116)

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

substrate/srml/democracy/src/lib.rs

@@ -358,11 +358,13 @@ mod tests {
 			transfer_fee: 0,
 			creation_fee: 0,
 			reclaim_rebate: 0,
+			_genesis_phantom_data: Default::default(),
 		}.build_storage().unwrap().0);
 		t.extend(GenesisConfig::<Test>{
 			launch_period: 1,
 			voting_period: 1,
 			minimum_deposit: 1,
+			_genesis_phantom_data: Default::default(),
 		}.build_storage().unwrap().0);
 		runtime_io::TestExternalities::new(t)
 	}