pay/sendpay: also store description in case bolt11 uses description_hash.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/libplugin-pay.h

@@ -259,6 +259,9 @@ struct payment {
 	 * by the invoice. */
 	const char *invstring;
 
+	/* Description, usually set if bolt11 has only description_hash */
+	const char *description;
+
 	/* If this is paying a local offer, this is the one (sendpay ensures we
 	 * don't pay twice for single-use offers) */
 	struct sha256 *local_offer_id;