ussd-dialog: Set phone input purose on entry

USSD menus are usually navigatable with dumb phone keyboards
so this makes a good default.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/761>

src/ui/main-window.ui

@@ -121,6 +121,7 @@
                 <child>
                   <object class="GtkEntry" id="ussd_entry">
                     <property name="visible">False</property>
+                    <property name="input-purpose">phone</property>
                     <signal name="changed" handler="window_ussd_entry_changed_cb" swapped="yes"/>
                   </object>
                 </child>