Add syntax files from upstream vim repository

syntax/docbkxml.vim

@@ -0,0 +1,11 @@
+if !exists('g:polyglot_disabled') || index(g:polyglot_disabled, 'vim') == -1
+  
+" Vim syntax file
+" Language:	DocBook XML
+" Maintainer:	Johannes Zellner <johannes@zellner.org>
+" Last Change:	Sam, 07 Sep 2002 17:20:12 CEST
+
+let b:docbk_type="xml"
+runtime syntax/docbk.vim
+
+endif