wire: derive_channel_id should accept const struct *bitcoin_txid

It's not modifying anything in the txid itself, just mashing it up
with the txout index.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

wire/fromwire.c

@@ -253,7 +253,7 @@ REGISTER_TYPE_TO_HEXSTR(channel_id);
  * (i.e. `funding_output_index` alters the last 2 bytes).
  */
 void derive_channel_id(struct channel_id *channel_id,
-		       struct bitcoin_txid *txid, u16 txout)
+		       const struct bitcoin_txid *txid, u16 txout)
 {
 	BUILD_ASSERT(sizeof(*channel_id) == sizeof(*txid));
 	memcpy(channel_id, txid, sizeof(*channel_id));

wire/wire.h

@@ -30,7 +30,7 @@ struct siphash_seed;
 typedef char wirestring;
 
 void derive_channel_id(struct channel_id *channel_id,
-		       struct bitcoin_txid *txid, u16 txout);
+		       const struct bitcoin_txid *txid, u16 txout);
 
 /* Read the type; returns -1 if not long enough.  cursor is a tal ptr. */
 int fromwire_peektype(const u8 *cursor);