Make sure KindlePowerD's __gc metamethod will actually be called

* Via the newproxy hack.

frontend/device/kindle/powerd.lua

@@ -125,13 +125,19 @@ function KindlePowerD:isChargingHW()
     return is_charging == 1
 end
 
-function KindlePowerD:__gc()
+local KindlePowerD_mt = {}
+function KindlePowerD_mt:__gc()
     if self.lipc_handle then
         self.lipc_handle:close()
         self.lipc_handle = nil
     end
 end
 
+-- Funky ass newproxy() workaround to make __gc work on a plain table in Lua 5.1/LuaJIT
+-- c.f., https://github.com/katlogic/__gc
+local setmetatable = require("ffi/__gc")
+setmetatable(KindlePowerD, KindlePowerD_mt)
+
 function KindlePowerD:_readFLIntensity()
     return self:read_int_file(self.fl_intensity_file)
 end