Seems commit 7b42d4feda makes GtkSpinner a direct subclass of GtkWidget but forgets to update the header file to include gtkwidget.h instead and declare the instance and class structures properly (assuming this was just a missed file in the commit).
7b42d4feda
Notify spinner::active only if it really changed an other cosmetic fixes
Add GtkSpinner activity throbber, as well as a cell renderer.