taglist: check that correct function exists

So 47bf6b1716 added support for
yegappan/taglist plugin, but unfortunately the old (deprecated)
vim-scripts/taglist.vim repo still exists and this one throws an error
when trying to access taglist#Tlist_Get_Tagname_By_Line

So check, that this function is defined before trying to load the
extension

autoload/airline/extensions/taglist.vim

@@ -4,7 +4,7 @@
 
 scriptencoding utf-8
 
-if !exists(':TlistShowTag')
+if !exists(':TlistShowTag') && !exists('*taglist#Tlist_Get_Tagname_By_Line')
   finish
 endif