Adds One ACCESSORY ONLY Slot for Helmets (#5573)

# About the pull request

Allows for the insertion of an extra cosmetic item in helmets that
subtype the M10.

# Explain why it's good for the game

Adds more personalization to people who don the helmet. Change is purely
cosmetic.


# 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
qol: Lets you put an extra accessory item in your helmet.
/:cl:

code/modules/clothing/head/helmet.dm

@@ -380,7 +380,7 @@ GLOBAL_LIST_INIT(allowed_helmet_items, list(
 
 	var/obj/item/storage/internal/headgear/pockets
 	var/storage_slots = 2 // keep in mind, one slot is reserved for garb items
-	var/storage_slots_reserved_for_garb = 1
+	var/storage_slots_reserved_for_garb = 2
 	var/storage_max_w_class = SIZE_TINY // can hold tiny items only, EXCEPT for glasses & metal flask.
 	var/storage_max_storage_space = 4