GTK_STYLE_CLASS_POPUP not defined in older gtks

Change-Id: I6192d13419aab42993e4b6dc011db30a4c3c1787
diff --git a/vcl/unx/gtk3/gtk3salnativewidgets-gtk.cxx b/vcl/unx/gtk3/gtk3salnativewidgets-gtk.cxx
index 11e6d01..3dcfb4a 100644
--- a/vcl/unx/gtk3/gtk3salnativewidgets-gtk.cxx
+++ b/vcl/unx/gtk3/gtk3salnativewidgets-gtk.cxx
@@ -1331,6 +1331,10 @@ GtkStyleContext* GtkSalGraphics::createNewContext(GtkControlPart ePart, gtk_widg
    return nullptr;
}

#ifndef GTK_STYLE_CLASS_POPUP
#define GTK_STYLE_CLASS_POPUP "popup"
#endif

GtkStyleContext* GtkSalGraphics::createOldContext(GtkControlPart ePart)
{
    switch (ePart)