st/icon-theme: Define default icon theme

In GTK, the value comes from another private header. It doesn't
make sense to copy it in its entirety, just define the one value
we actually need ourselves.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2620>

src/st/st-icon-theme.c

@@ -47,7 +47,6 @@
 #include "gtkintl.h"
 #include "gtkmain.h"
 #include "deprecated/gtknumerableiconprivate.h"
-#include "gtksettingsprivate.h"
 #include "gtkstylecontextprivate.h"
 
 #undef GDK_DEPRECATED
@@ -62,6 +61,8 @@
  */
 #include "fallback-c89.c"
 
+#define DEFAULT_ICON_THEME "Adwaita"
+
 /**
  * SECTION:gtkicontheme
  * @Short_description: Looking up icons by name