gtk/tests: Fix for -Wmissing-declarations

I'm adding a bunch of fixes for gcc complaining about
-Wmissing-declarations after finding a bunch of cases today where I
had forgotten to make functions static in the CSS code.

This patch fixes the tests in gtk/tests.

After this last patch, the gtk/ subdir should now compile without
warnings when this flag is enabled.

gtk/tests/treestore.c

@@ -26,6 +26,8 @@
  *  - And we also need tests for creating these child levels, etc.
  */
 
+#include "treemodel.h"
+
 #include <gtk/gtk.h>
 
 static inline gboolean