add "!" to function signatures

autoload/airline/async.vim

@@ -123,7 +123,7 @@ if v:version >= 800 && has("job")
     let s:po_jobs[a:file] = id
   endfunction
 
-  function airline#async#vim_vcs_untracked(config, file)
+  function! airline#async#vim_vcs_untracked(config, file)
     if g:airline#init#is_windows && &shell =~ 'cmd'
       let cmd = a:config['cmd'] . shellescape(a:file)
     else