Introduce a PhantomData field in GenesisConfig (#1116)

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

substrate/srml/example/src/lib.rs

@@ -313,6 +313,7 @@ mod tests {
 		t.extend(GenesisConfig::<Test>{
 			dummy: 42,
 			foo: 24,
+			_genesis_phantom_data: Default::default(),
 		}.build_storage().unwrap().0);
 		t.into()
 	}