Adds the ability to put Folding Sheilds in the IMP ammo rack (#4470)

# About the pull request

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--> Adds the folding shield to the ammo rack to give it a bit more of a
use, also seemed wanted

![Discord_ksdl2muR76](https://github.com/cmss13-devs/cmss13/assets/30062054/3905c481-18c4-41e5-b6bb-e0fee693c9e0)

# Explain why it's good for the game
It Gives a bit more use of the IMP ammo rack, and is just a cool feature
in general, more utility for a under used item!
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>


![image](https://github.com/cmss13-devs/cmss13/assets/30062054/1cb3335f-8f08-47ed-943e-ad9bbe74e946)

</details>


# Changelog
:cl:
add: Added the ability for the IMP Ammo Rack to hold folding barricades.

/:cl:

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

@@ -410,7 +410,7 @@
 	has_gamemode_skin = FALSE
 	storage_slots = 3
 	icon_state = "ammo_pack_0"
-	can_hold = list(/obj/item/ammo_box)
+	can_hold = list(/obj/item/ammo_box, /obj/item/stack/folding_barricade)
 	max_w_class = SIZE_MASSIVE
 	throw_range = 0
 	xeno_types = null