Add syntax files from upstream vim repository

compiler/xmllint.vim

@@ -0,0 +1,30 @@
+if !exists('g:polyglot_disabled') || index(g:polyglot_disabled, 'vim') == -1
+  
+" Vim compiler file
+" Compiler:	xmllint
+" Maintainer:	Doug Kearns <dougkearns@gmail.com>
+" Last Change:	2013 Jul 8
+
+if exists("current_compiler")
+  finish
+endif
+let current_compiler = "xmllint"
+
+if exists(":CompilerSet") != 2		" older Vim always used :setlocal
+  command -nargs=* CompilerSet setlocal <args>
+endif
+
+let s:cpo_save = &cpo
+set cpo-=C
+
+CompilerSet makeprg=xmllint\ --valid\ --noout
+
+CompilerSet errorformat=%+E%f:%l:\ %.%#\ error\ :\ %m,
+		    \%+W%f:%l:\ %.%#\ warning\ :\ %m,
+		    \%-Z%p^,
+		    \%-G%.%#
+
+let &cpo = s:cpo_save
+unlet s:cpo_save
+
+endif