gnome3.gnome-settings-daemon: add cflag G_DISABLE_CAST_CHECKS

This is what would have been passed before with the release
buildtype.

See: https://gitlab.gnome.org/GNOME/gnome-settings-daemon/blob/GNOME_SETTINGS_DAEMON_3_32_1/meson.build#L73

pkgs/desktops/gnome-3/core/gnome-settings-daemon/default.nix

@@ -95,6 +95,12 @@ stdenv.mkDerivation rec {
     "-Dudev_dir=${placeholder "out"}/lib/udev"
   ];
 
+  NIX_CFLAGS_COMPILE = [
+    # Default for release buildtype but passed manually because
+    # we're using plain
+    "-DG_DISABLE_CAST_CHECKS"
+  ];
+
   # So the polkit policy can reference /run/current-system/sw/bin/gnome-settings-daemon/gsd-backlight-helper
   postFixup = ''
     mkdir -p $out/bin/gnome-settings-daemon