luacheck: configure it for .luacheckrc itself

.luacheckrc

@@ -10,6 +10,9 @@ files["spec"].std = "+busted"
 -- The default config may set global variables
 files["awesomerc.lua"].allow_defined_top = true
 
+-- This file itself
+files[".luacheckrc"].ignore = {"111", "112", "131"}
+
 -- Global objects defined by the C code
 read_globals = {
     "awesome",