tx: expose elements 'is fee' calculation

we'll use it when figuring out what outputs to account for during
a withdrawal.

bitcoin/tx.c

@@ -63,7 +63,7 @@ int bitcoin_tx_add_multi_outputs(struct bitcoin_tx *tx,
 	return tx->wtx->num_outputs;
 }
 
-static bool elements_tx_output_is_fee(const struct bitcoin_tx *tx, int outnum)
+bool elements_tx_output_is_fee(const struct bitcoin_tx *tx, int outnum)
 {
 	assert(outnum < tx->wtx->num_outputs);
 	return chainparams->is_elements &&

bitcoin/tx.h

@@ -172,4 +172,9 @@ bool bitcoin_tx_check(const struct bitcoin_tx *tx);
  */
 void bitcoin_tx_finalize(struct bitcoin_tx *tx);
 
+/**
+ * Returns true if the given outnum is a fee output
+ */
+bool elements_tx_output_is_fee(const struct bitcoin_tx *tx, int outnum);
+
 #endif /* LIGHTNING_BITCOIN_TX_H */