Fixes missing pipe shotgun sprite. (shiptest-ss13#2729)

## About The Pull Request

Tin

## Why It's Good For The Game

Oops.

## Changelog

:cl:
fix: fixes invisble pipe shotguns
/:cl:

code/modules/projectiles/guns/ballistic/shotgun.dm

@@ -378,6 +378,7 @@
 /obj/item/gun/ballistic/shotgun/doublebarrel/improvised
 	name = "improvised shotgun"
 	desc = "A length of pipe and miscellaneous bits of scrap fashioned into a rudimentary single-shot shotgun."
+	icon = 'icons/obj/guns/projectile.dmi'
 	base_icon_state = "ishotgun"
 	icon_state = "ishotgun"
 	item_state = "ishotgun"