mpp: Enable adaptive splitter

Changelog-Added: The adaptive multi-part payment modifier will split payments that are failing due to their size into smaller parts, and re-attempted.

plugins/pay.c

@@ -1847,6 +1847,7 @@ struct payment_modifier *paymod_mods[] = {
 	&routehints_pay_mod,
 	&waitblockheight_pay_mod,
 	&retry_pay_mod,
+	&adaptive_splitter_pay_mod,
 	NULL,
 };