tdf#123031 allow natural cell width if the combobox contents are >= request

which matters in at least gtk 3.19

Change-Id: I760e46d02f2678842a685aa077bd0129520a3263
Reviewed-on: https://gerrit.libreoffice.org/67074
Tested-by: Jenkins
Reviewed-by: Xisco Faulí <xiscofauli@libreoffice.org>

vcl/unx/gtk3/gtk3gtkinst.cxx

@@ -6943,7 +6943,7 @@ public:
         else
         {
             g_object_set(G_OBJECT(m_pTextRenderer), "ellipsize", PANGO_ELLIPSIZE_NONE, nullptr);
-            gtk_cell_renderer_set_fixed_size(cell, size.width, size.height);
+            gtk_cell_renderer_set_fixed_size(cell, -1, size.height);
         }
         g_list_free(cells);