Fixed Meta disposals. (#78819)

## About The Pull Request

Fixes #78811

The sorting helper for disposals on Meta was missing, causing garbage to
infinitely loop around the station instead of being thrown into
disposals. This has been rectified.
## Why It's Good For The Game

Infinite disposals loops are bad unless they're on purpose.
## Changelog
🆑
fix: Metastation disposals will no longer infinitely loop garbage around
the station.
/🆑

_maps/map_files/MetaStation/MetaStation.dmm

@@ -18503,6 +18503,7 @@
 /obj/structure/disposalpipe/sorting/mail/flip{
 	dir = 1
 	},
+/obj/effect/mapping_helpers/mail_sorting/supply/disposals,
 /turf/open/floor/plating,
 /area/station/maintenance/port/fore)
 "gQw" = (