selection: Disable deprecation warnings

We are using a lot of deprecated API, and we know it.

Since the selection code is going to be replaced in GTK 4.0, there's no
real point in keeping the warnings enabled in 3.22.

gtk/gtkselection.c

@@ -75,6 +75,9 @@
  * associated information.
  */
 
+/* We are using deprecated API, here, and we know that */
+#define GDK_DISABLE_DEPRECATION_WARNINGS
+
 #include "config.h"
 
 #include "gtkselection.h"