expat moved to github

see https://sourceforge.net/p/expat/code_git/ci/master/tree/MIGRATED_TO_GITHUB.txt

Change-Id: I9f0351debb69f7aae2b066ad90e983cb6259bff5
Reviewed-on: https://gerrit.libreoffice.org/34041Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

external/expat/README

 Simple SAX parser library with added UTF-16 support.
 
 From:
-[http://expat.sourceforge.net/]
+[https://github.com/libexpat/libexpat]