Disable Kerberos support, we don't have requests_gssapi or requests_kerberos

Index: Nagstamon/QUI/__init__.py
--- Nagstamon/QUI/__init__.py.orig
+++ Nagstamon/QUI/__init__.py
@@ -5807,7 +5807,7 @@ class Dialog_Server(Dialog):
         self.ui.button_choose_custom_cert_ca_file.clicked.connect(self.choose_custom_cert_ca_file)
 
         # fill authentication combobox
-        self.ui.input_combobox_authentication.addItems(['Basic', 'Digest', 'Kerberos'])
+        self.ui.input_combobox_authentication.addItems(['Basic', 'Digest'])
         if ECP_AVAILABLE is True:
             self.ui.input_combobox_authentication.addItems(['ECP'])