gnome3.gnome-shell: Fix screen recorder

Trying to record screen (ctr-alt-shift-r) fails with the following logged:

	Can't create videoconvert element
	shell_recorder_close: assertion 'recorder->state != RECORDER_STATE_CLOSED' failed

videoconvert plug-in is part of gst-plugins-base.

pkgs/desktops/gnome-3/core/gnome-shell/default.nix

@@ -43,6 +43,7 @@ in stdenv.mkDerivation rec {
 
     # recording
     gst_all_1.gstreamer
+    gst_all_1.gst-plugins-base
     gst_all_1.gst-plugins-good
 
     # not declared at build time, but typelib is needed at runtime