missing standard border-width

Change-Id: I4aa1fd132e3d6e153bdc03a51bf81469301eb1fb
Reviewed-on: https://gerrit.libreoffice.org/68169
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

cui/uiconfig/ui/eventassignpage.ui

@@ -37,6 +37,7 @@
     <property name="can_focus">False</property>
     <property name="hexpand">True</property>
     <property name="vexpand">True</property>
+    <property name="border_width">6</property>
     <property name="row_spacing">12</property>
     <property name="column_spacing">12</property>
     <property name="row_homogeneous">True</property>