Flatpak: Fix access to GnuPG keys

Fixes gajim/gajim-plugins#386

flatpak/org.gajim.Gajim.yaml

@@ -20,6 +20,8 @@ finish-args:
   - --filesystem=~/.config/dconf:ro
   - --talk-name=ca.desrt.dconf
   - --env=DCONF_USER_CONFIG_DIR=.config/dconf
+  # GnuPG
+  - --filesystem=~/.gnupg
   # extensions
   - --env=PYTHONPATH=/app/plugins/lib/python3.7/site-packages