add missing libxml2 dependencies in GLib easyconfigs

[boegel]

builds on top of #2638 by @nathanhaigh, fixes conflict on libxml2 version in DOLFIN dependency tree

[hpcugentbot]

Easyconfigs unit test suite FAILed.

See https://jenkins1.ugent.be/job/easybuild-easyconfigs-pr-builder/6872/console for more details.

Please fix the reported issues by pushing additional commits to the branch corresponding with this pull request; contact @boegel if you're not sure what to do.

[hpcugentbot]

Easyconfigs unit test suite FAILed.

See https://jenkins1.ugent.be/job/easybuild-easyconfigs-pr-builder/6881/console for more details.

Please fix the reported issues by pushing additional commits to the branch corresponding with this pull request; contact @boegel if you're not sure what to do.

[hpcugentbot]

Easyconfigs unit test suite PASSed (see https://jenkins1.ugent.be/job/easybuild-easyconfigs-pr-builder/6884/console for more details).

This pull request is now ready for review/testing.

Please try and find someone who can tackle this; contact @boegel if you're not sure what to do.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 14 out of 14 (14 easyconfigs in this PR)
node2090.delcatty.os - Linux SL 6.7, Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz, Python 2.6.6
See https://gist.github.com/76a42dde59512f12e263 for a full test report.

[boegel]

going in as reviewed version of #2638, thanks @nathanhaigh!