Bug 1609898, fix xml parsing error introduced by bug 1605467, r=Gijs

This is just a bustage fix, and doesn't try to address the question
of test coverage.

Differential Revision: https://phabricator.services.mozilla.com/D60234

--HG--
extra : moz-landing-system : lando

browser/components/downloads/content/contentAreaDownloadsView.xhtml

@@ -23,6 +23,7 @@
    -->
 <window id="contentAreaDownloadsView"
         xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        xmlns:html="http://www.w3.org/1999/xhtml"
         title="&downloads.title;"
         csp="default-src chrome:; script-src chrome: 'sha512-4o5Uf4E4EG+90Mb820FH2YFDf4IuX4bfUwQC7reK1ZhgcXWJBKMK2330XIELaFJJ8HiPffS9mP60MPjuXMIrHA=='; img-src chrome: moz-icon:; object-src 'none'">