main-window.ui: add a proper callback for Close button

The signal click of Close button is connected to "gtk_widget_hide".
As a result the window is hided, but still active and the whole
program renders unresponsive.

The Close button can appear for example if USSD_STATUS_IDLE comes
before a user press Cancel button in dialog window
(could be for example because of carrier timeout).
The "adw_dialog_close" function should be called instead.

Signed-off-by: knoppix <knoppix@4d2.org>
Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/776>

src/calls-main-window.c

@@ -201,7 +201,6 @@ window_ussd_cancel_clicked_cb (CallsMainWindow *self)
   adw_dialog_close (self->ussd_dialog);
 }
 
-
 static void
 window_ussd_entry_changed_cb (CallsMainWindow *self,
                               GtkEntry        *entry)