build: Don't use an intermediate .pot file to do translations

Declare freedesktop.org.xml.in to be translatable directly, without
extracting translatable strings from a .pot which we then translate,
which we then need to merge into translations.

Files in its/ from Robert Ancell <robert.ancell@canonical.com>
!4
1 job for wip/hadess/really-gettext in 4 minutes and 53 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  Test
passed #588202
build

00:04:53