don't update the gtk icon cache

Index: artwork/Makefile.in
--- artwork/Makefile.in.orig
+++ artwork/Makefile.in
@@ -726,7 +726,7 @@ uninstall-am: uninstall-dist_appicons128DATA \
 
 
 update-icon-cache:
-	which gtk-update-icon-cache >/dev/null && gtk-update-icon-cache -f -t $(DESTDIR)$(iconsdir) || true
+	true
 
 install-data-hook: update-icon-cache
 uninstall-hook: update-icon-cache
