pncconf: restore translation

I didn't investigate closely, but it looks like this line was
overlooked when transitioning to the MultiFileBuilder.

.. after this, running with LANG=fr_FR.UTF-8 again translates
the main buttons.  (Many other texts aren't translated, but this
seems to be because the translation is out of date)

src/emc/usr_intf/pncconf/pncconf.py

@@ -1406,6 +1406,7 @@ def __init__(self, dbgstate=0):
         bar_size = 0
         # build the glade files
         self.builder = MultiFileBuilder()
+        self.builder.set_translation_domain(domain)
         self.builder.add_from_file(os.path.join(self._p.DATADIR,'main_page.glade'))
         self.builder.add_from_file(os.path.join(self._p.DATADIR,'dialogs.glade'))
         self.builder.add_from_file(os.path.join(self._p.DATADIR,'help.glade'))