fix: make offset sight rail spawn (#60738)

there are a surprising amount of items that do not spawn at all, this
one is of them

add it to a common gunmod spawn group, should be good enough

now that I thing about it this a "common" gun mod that allows for
multiple sights, great!

data/json/itemgroups/Weapons_Mods_Ammo/gunmod.json

@@ -14,6 +14,7 @@
       [ "butt_hook", 30 ],
       [ "match_trigger", 20 ],
       [ "offset_sights", 40 ],
+      [ "offset_sight_rail", 30 ],
       [ "cheek_pad", 25 ],
       [ "rail_laser_sight", 40 ],
       [ "red_dot_sight", 70 ],