Resolves: rhbz#1408500 gtk3 toolbar borders too distinct vs native gtk3 ones

Change-Id: I7ce284d3e6f999e2ac08326dc40b060bf70d7d4b

vcl/unx/gtk3/gtk3salnativewidgets-gtk.cxx

@@ -2927,6 +2927,7 @@ void GtkData::initNWF()
     ImplSVData* pSVData = ImplGetSVData();
     pSVData->maNWFData.mbFlatMenu = true;
     pSVData->maNWFData.mbCheckBoxNeedsErase = true;
+    pSVData->maNWFData.mbDockingAreaAvoidTBFrames = true;
     pSVData->maNWFData.mbCanDrawWidgetAnySize = true;
     pSVData->maNWFData.mbDDListBoxNoTextArea = true;
     pSVData->maNWFData.mbNoFocusRects = true;