Force setting of filetype

In combination with certain plugins, the filetype wouldn't be set for
Startify, because a filetype was already set by another plugin in the
series of VimEnter autocmds.

Setting the filetype gets forced now.

autoload/startify.vim

@@ -48,7 +48,7 @@ function! startify#insane_in_the_membrane() abort
 
   setlocal noswapfile nobuflisted buftype=nofile bufhidden=wipe
   setlocal nonumber nocursorline nolist statusline=\ startify
-  setfiletype startify
+  set filetype=startify
 
   if v:version >= 703
     setlocal norelativenumber