wordcount: work-around non-existing v:t_list

v:t_list is an internal variable describing the type of a list.
Unfortunately, this is not available in older vims. So let's fall back
to using `type([])`

see #1795

autoload/airline/extensions/wordcount.vim

@@ -89,7 +89,7 @@ function! airline#extensions#wordcount#apply(...)
     let s:filetypes = filetypes
 
     " Select test based on type of "filetypes": new=list, old=string
-    if type(filetypes) == v:t_list
+    if type(filetypes) == get(v:, 't_list', type([]))
           \ ? index(filetypes, &filetype) > -1 || index(filetypes, 'all') > -1
           \ : match(&filetype, filetypes) > -1
       let b:airline_changedtick = -1