gnu: expat: fix CVE-2018-20843

* gnu/packages/xml.scm (expat)[replacement]: New field.
(expat/fixed): New variable.
* gnu/packages/patches/expat-CVE-2018-20843.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add patch file.

Signed-off-by: Marius Bakke <mbakke@fastmail.com>