Fix wxGLCanvas compilation with wxUSE_PALETTE==0.

This fixes compilation problems with the minimal build of wxGTK and will make
removing palette support in the future simpler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66088 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/glcanvas.cpp

@@ -203,9 +203,12 @@ bool wxGLCanvas::Create(wxWindow *parent,
                         long style,
                         const wxString& name,
                         const int *attribList,
-                        const wxPalette& WXUNUSED_UNLESS_DEBUG(palette))
+                        const wxPalette& palette)
 {
+#if wxUSE_PALETTE
     wxASSERT_MSG( !palette.IsOk(), wxT("palettes not supported") );
+#endif // wxUSE_PALETTE
+    wxUnusedVar(palette); // Unused when wxDEBUG_LEVEL==0
 
     m_exposed = false;
     m_noExpose = true;