Make collator generate the correct block data

cumulus/runtime/src/lib.rs

@@ -41,6 +41,7 @@ pub struct ParachainBlockData<B: BlockT> {
 	extrinsics: Vec<<B as BlockT>::Extrinsic>,
 	/// The data that is required to emulate the storage accesses executed by all extrinsics.
 	witness_data: WitnessData,
+	/// The storage root of the witness data.
 	witness_data_storage_root: <B as BlockT>::Hash,
 }
 
@@ -58,4 +59,9 @@ impl<B: BlockT> ParachainBlockData<B> {
 			witness_data_storage_root,
 		}
 	}
-}
+
+	/// Convert `self` into the stored header.
+	pub fn into_header(self) -> B::Header {
+		self.header
+	}
+}