psbt: use pset initializer if needed

we're moving over to elements!

bitcoin/psbt.c

@@ -1,4 +1,5 @@
 #include <assert.h>
+#include <bitcoin/chainparams.h>
 #include <bitcoin/psbt.h>
 #include <bitcoin/pubkey.h>
 #include <bitcoin/script.h>
@@ -47,7 +48,10 @@ struct wally_psbt *new_psbt(const tal_t *ctx, const struct wally_tx *wtx)
 	size_t *script_lens;
 	struct wally_tx_witness_stack **witnesses;
 
-	wally_err = wally_psbt_init_alloc(wtx->num_inputs, wtx->num_outputs, 0, &psbt);
+	if (is_elements(chainparams))
+		wally_err = wally_psbt_elements_init_alloc(wtx->num_inputs, wtx->num_outputs, 0, &psbt);
+	else
+		wally_err = wally_psbt_init_alloc(wtx->num_inputs, wtx->num_outputs, 0, &psbt);
 	assert(wally_err == WALLY_OK);
 	tal_add_destructor(psbt, psbt_destroy);