tdf#106127: Sidebar: Enable patterns as page background

don't know who in their right mind would want to use it, but it's
possible to set such background in the dialogs, so let's make this
consistent

Change-Id: I0d5dea0edfeae8d8ca65ad2bd18ced9a982a224c
Reviewed-on: https://gerrit.libreoffice.org/37265Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

sw/source/uibase/sidebar/PageStylesPanel.hxx

@@ -111,6 +111,7 @@ private:
     XGradient GetGradientSetOrDefault();
     const OUString GetHatchingSetOrDefault();
     const OUString GetBitmapSetOrDefault();
+    const OUString GetPatternSetOrDefault();
 
     void ModifyFillColor();