ensure gtk creates a default label widget for the MenuButtons

Change-Id: Id8b6f11b4429c8029cacfc26592767625efb9674
Reviewed-on: https://gerrit.libreoffice.org/70798
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

cui/uiconfig/ui/gradientpage.ui

@@ -454,6 +454,7 @@
                         <property name="receives_default">False</property>
                         <property name="xalign">0</property>
                         <property name="draw_indicator">True</property>
+                        <property name="label" translatable="no"></property>
                         <child>
                           <placeholder/>
                         </child>
@@ -485,6 +486,7 @@
                         <property name="receives_default">False</property>
                         <property name="xalign">0</property>
                         <property name="draw_indicator">True</property>
+                        <property name="label" translatable="no"></property>
                         <child>
                           <placeholder/>
                         </child>