firewall-config: Add missing gettext.textdomain call to fix translations

gettext.gettext was used without setting the textdomain. This resulted in failed
translations and also in a traceback in the rich rule dialog while adding a
service or port, ..

The failed translations only happended for strings that have been translated
using _() in the code, but not in glade.

src/firewall-config

@@ -23,8 +23,6 @@
 import sys
 import string
 import gi
-import gettext
-_ = gettext.gettext
 
 try:
     gi.require_version('Gtk', '3.0')
@@ -57,6 +55,10 @@ from firewall.core.io.ipset import IPSet
 from firewall.core import rich
 from firewall.core.fw_nm import *
 
+import gettext
+gettext.textdomain(config.DOMAIN)
+_ = gettext.gettext
+
 if not datadir:
     datadir = config.DATADIR
     sys.path.insert(0, datadir)