Allows the MOU53 to be stored in the shotgun scabbard. (#5705)

# About the pull request

Lets you store the MOU in the shotgun scabbard.

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# Explain why it's good for the game

Shotgun scabbard is soul. Basically no advantage given, other than the
fact that you can keep a slot reserved for your MOU.
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl: Stakeyng
add: mou is now compatible with the shotgun scabbard
/:cl:

code/game/objects/items/storage/large_holster.dm

@@ -64,6 +64,7 @@
 	can_hold = list(
 		/obj/item/weapon/gun/shotgun/pump,
 		/obj/item/weapon/gun/shotgun/combat,
+		/obj/item/weapon/gun/shotgun/double/mou53,
 	)
 	has_gamemode_skin = TRUE