smpeg: fix build by linking against libX11 explicitly

Error:

/nix/store/anvi6cx0rcj9xn48af849qnr590avyn3-binutils-2.30/bin/ld: gtv.o: undefined reference to symbol 'XMoveWindow'
/nix/store/ps571bzx27x5imzifw4wiah32mpqpgva-libX11-1.6.7/lib/libX11.so.6: error adding symbols: DSO missing from command line

pkgs/development/libraries/smpeg/default.nix

@@ -39,6 +39,8 @@ stdenv.mkDerivation rec {
       --prefix PKG_CONFIG_PATH ":" "${SDL.dev}/lib/pkgconfig"
   '';
 
+  NIX_LDFLAGS = [ "-lX11" ];
+
   meta = {
     homepage = http://icculus.org/smpeg/;
     description = "MPEG decoding library";