gtk: Add missing gsk/v4_20 feature

Author: sdroege

gtk4/Cargo.toml

@@ -26,7 +26,7 @@ v4_12 = ["gtk4-sys/v4_12", "v4_10", "gdk/v4_12"]
 v4_14 = ["gtk4-sys/v4_14", "v4_12", "gdk/v4_14", "gsk/v4_14"]
 v4_16 = ["gtk4-sys/v4_16", "v4_14", "gdk/v4_16", "gsk/v4_16"]
 v4_18 = ["gtk4-sys/v4_18", "gdk/v4_18", "v4_16"]
-v4_20 = ["gtk4-sys/v4_20", "gdk/v4_20", "v4_18"]
+v4_20 = ["gtk4-sys/v4_20", "gdk/v4_20", "gsk/v4_20", "v4_18"]
 gio_v2_80 = ["gio/v2_80"]
 xml_validation = ["gtk4-macros/xml_validation"]
 blueprint = ["gtk4-macros/blueprint"]