Fixes humans unable to use vehicle gunner seat (#6247)

# About the pull request

Fixes #6246

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# Explain why it's good for the game

Vehicle gunnery is meant to be used by humans.
# 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:
fix: humans can use vehicle gunner seats again
/:cl:

code/__DEFINES/traits.dm

@@ -354,6 +354,7 @@ GLOBAL_LIST_INIT(traits_by_type, list(
 	/mob/living/carbon/xenomorph = list(
 		"TRAIT_ABILITY_NO_PLASMA_TRANSFER" = TRAIT_ABILITY_NO_PLASMA_TRANSFER,
 		"TRAIT_ABILITY_OVIPOSITOR" = TRAIT_ABILITY_OVIPOSITOR,
+		"TRAIT_OPPOSABLE_THUMBS" = TRAIT_OPPOSABLE_THUMBS,
 	),
 	/datum/hive_status = list(
 		"TRAIT_XENONID" = TRAIT_XENONID,

code/game/objects/objs.dm

@@ -306,6 +306,7 @@
 	if (M.mob_size <= MOB_SIZE_XENO)
 		if ((M.stat == DEAD && istype(src, /obj/structure/bed/roller) || HAS_TRAIT(M, TRAIT_OPPOSABLE_THUMBS)))
 			do_buckle(M, user)
+			return
 	if ((M.mob_size > MOB_SIZE_HUMAN))
 		to_chat(user, SPAN_WARNING("[M] is too big to buckle in."))
 		return

code/modules/vehicles/interior/interactable/seats.dm

@@ -108,8 +108,6 @@
 				to_chat(user, SPAN_WARNING("You are unable to use heavy weaponry."))
 			return
 
-		if(!HAS_TRAIT(user, TRAIT_OPPOSABLE_THUMBS))
-			return
 	for(var/obj/item/I in user.contents)		//prevents shooting while zoomed in, but zoom can still be activated and used without shooting
 		if(I.zoom)
 			I.zoom(user)