Open funcs_data.mpack in binary mode. (#14944)

"b" flag is required to read binary files on Windows. 
https://www.lua.org/pil/21.2.2.html

scripts/genvimvim.lua

@@ -123,7 +123,7 @@ end
 w('\n\nsyn case match')
 local vimfun_start = 'syn keyword vimFuncName contained '
 w('\n\n' .. vimfun_start)
-funcs = mpack.unpack(io.open(funcs_file):read("*all"))
+funcs = mpack.unpack(io.open(funcs_file, 'rb'):read("*all"))
 local started = 0
 for name, def in pairs(funcs) do
   if name then