util.lua: fix lfs require in isEmptyDir()

Pointed out by @poire-z https://github.com/koreader/koreader/commit/ed7937f65d0891f54491be8f77f9864edaaad460#commitcomment-21956632

frontend/util.lua

@@ -287,7 +287,8 @@ end
 ---- @string path
 ---- @treturn bool
 function util.isEmptyDir(path)
-    for filename in require("lfs").dir(path) do
+    local lfs = require("libs/libkoreader-lfs")
+    for filename in lfs.dir(path) do
         if filename ~= '.' and filename ~= '..' then
             return false
         end