libvdpau: add -lX11 on darwin

Seems to be necessary for some reason. Otherwise we are missing
symbols.

pkgs/development/libraries/libvdpau/default.nix

···
   configureFlags = stdenv.lib.optional stdenv.isLinux
     "--with-module-dir=${libGL_driver.driverLink}/lib/vdpau";
 
+  NIX_LDFLAGS = if stdenv.isDarwin then "-lX11" else null;
+
   installFlags = [ "moduledir=$(out)/lib/vdpau" ];
 
   meta = with stdenv.lib; {