fmf: Add glibc-langpack-de test dependency

The tests switch to German, make sure that the corresponding locale
exists. Otherwise, tests will run into unexpected messages like "invalid
or unusable locale: de_DE.UTF-8".

test/browser/main.fmf

@@ -5,6 +5,7 @@ require:
   - cockpit-ws
   - cockpit-system
   - git
+  - glibc-langpack-de
   - libvirt-python3
   - make
   - npm