Pocketbook frontend: Fix up missing require. (#6604)

This one slipped through tests somehow.

frontend/device/pocketbook/device.lua

@@ -4,6 +4,7 @@ local ffi = require("ffi")
 local C = ffi.C
 local inkview = ffi.load("inkview")
 local band = require("bit").band
+local util = require("util")
 
 require("ffi/posix_h")
 require("ffi/linux_input_h")