Fix return type of GtkSpinButton "output" signal handler

This doesn't seem to change anything in practice, but still use the
correct return type.

Co-Authored-By: Paul Cornett <paulcor@users.noreply.github.com>

src/gtk/spinctrl.cpp

@@ -88,7 +88,7 @@ wx_gtk_spin_input(GtkSpinButton*, gdouble* val, wxSpinCtrlGTKBase* win)
     return FALSE;
 }
 
-static gint
+static gboolean
 wx_gtk_spin_output(GtkSpinButton* spin, wxSpinCtrlGTKBase* win)
 {
     wxString text;