battery_box.lua: Make "srcstack" local (#525)

fgrosswig

2020-01-06 e90b28895cc28daf1a2e4fb772bd191400cf9e62

 technic/tools/vacuum.lua

@@ -24,7 +24,7 @@
            gain = 0.4,
         })
      end
      local pos = user:getpos()
      local pos = user:get_pos()
      local inv = user:get_inventory()
      for _, object in ipairs(minetest.get_objects_inside_radius(pos, vacuum_range)) do
         local luaentity = object:get_luaentity()