Set OK as default button in MM Assign Fields dialog

Shut this warning up:
warn:vcl.layout:28925:1:vcl/source/window/builder.cxx:519:
No default button defined in modules/swriter/ui/assignfieldsdialog.ui

Change-Id: Ide113c694ac6cf20361a81ed53c474494f172921
Reviewed-on: https://gerrit.libreoffice.org/39190Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

sw/uiconfig/swriter/ui/assignfieldsdialog.ui

@@ -23,6 +23,7 @@
                 <property name="use_action_appearance">False</property>
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
+                <property name="has_default">True</property>
                 <property name="receives_default">True</property>
                 <property name="use_stock">True</property>
               </object>