Update to latest libxml2

- Update from libxml2 2.9.8 to 2.9.9
- Cannot convert to build from git because autoconf 2.69
  is required and CentOS 6 only provides up to 2.68

Closes: https://gitlab.com/gitlab-org/distribution/team-tasks/issues/302



Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

changelogs/unreleased/302-update-libxml2.yml

+---
+title: Update to latest libxml2
+merge_request: 3156
+author:
+type: other

config/software/libxml2.rb

@@ -15,7 +15,7 @@
 #
 name 'libxml2'
-default_version '2.9.8'
+default_version '2.9.9'
 license 'MIT'
 license_file 'COPYING'
@@ -27,8 +27,8 @@ dependency 'libiconv'
 dependency 'liblzma'
 dependency 'config_guess'
-version '2.9.8' do
-  source md5: 'b786e353e2aa1b872d70d5d1ca0c740d'
+version '2.9.9' do
+  source md5: 'c04a5a0a042eaa157e8e8c9eabe76bd6'
 end
 source url: "ftp://xmlsoft.org/libxml2/libxml2-#{version}.tar.gz"