psbt: hoist up `psbt_add_serials`, so we can use it elsewhere

We're going to use this in multifundchannel.

common/psbt_open.c

@@ -584,3 +584,25 @@ bool psbt_side_finalized(const struct wally_psbt *psbt, enum tx_role role)
 	}
 	return true;
 }
+
+/* Adds serials to inputs + outputs that don't have one yet */
+void psbt_add_serials(struct wally_psbt *psbt, enum tx_role role)
+{
+	u16 serial_id;
+	for (size_t i = 0; i < psbt->num_inputs; i++) {
+		/* Skip ones that already have a serial id */
+		if (psbt_get_serial_id(&psbt->inputs[i].unknowns, &serial_id))
+			continue;
+
+		serial_id = psbt_new_input_serial(psbt, role);
+		psbt_input_set_serial_id(psbt, &psbt->inputs[i], serial_id);
+	}
+	for (size_t i = 0; i < psbt->num_outputs; i++) {
+		/* Skip ones that already have a serial id */
+		if (psbt_get_serial_id(&psbt->outputs[i].unknowns, &serial_id))
+			continue;
+
+		serial_id = psbt_new_output_serial(psbt, role);
+		psbt_output_set_serial_id(psbt, &psbt->outputs[i], serial_id);
+	}
+}

common/psbt_open.h

@@ -183,4 +183,15 @@ psbt_to_witness_stacks(const tal_t *ctx,
 /* psbt_side_finalized - True if designated role has all signature data */
 bool psbt_side_finalized(const struct wally_psbt *psbt,
 			 enum tx_role role);
+
+/* psbt_add_serials - Add serials to inputs/outputs that are missing them
+ *
+ * Adds a serial of the correct parity for the designated {role} to all
+ * inputs and outputs of this PSBT that do not currently have a serial_id
+ * set.
+ *
+ * @psbt - the psbt to add serials to
+ * @role - the role we should use to select serial parity
+ */
+void psbt_add_serials(struct wally_psbt *psbt, enum tx_role role);
 #endif /* LIGHTNING_COMMON_PSBT_OPEN_H */

lightningd/dual_open_control.c

@@ -332,28 +332,6 @@ static bool psbt_side_contribs_changed(struct wally_psbt *orig,
 	return false;
 }
 
-/* Adds serials to inputs + outputs that don't have one yet */
-static void psbt_add_serials(struct wally_psbt *psbt, enum tx_role role)
-{
-	u16 serial_id;
-	for (size_t i = 0; i < psbt->num_inputs; i++) {
-		/* Skip ones that already have a serial id */
-		if (psbt_get_serial_id(&psbt->inputs[i].unknowns, &serial_id))
-			continue;
-
-		serial_id = psbt_new_input_serial(psbt, role);
-		psbt_input_set_serial_id(psbt, &psbt->inputs[i], serial_id);
-	}
-	for (size_t i = 0; i < psbt->num_outputs; i++) {
-		/* Skip ones that already have a serial id */
-		if (psbt_get_serial_id(&psbt->outputs[i].unknowns, &serial_id))
-			continue;
-
-		serial_id = psbt_new_output_serial(psbt, role);
-		psbt_output_set_serial_id(psbt, &psbt->outputs[i], serial_id);
-	}
-}
-
 /* dualopend dies?  Remove dualopend ptr from payload */
 static void openchannel2_remove_dualopend(struct subd *dualopend,
 					  struct openchannel2_payload *payload)