qt5Libs.phonon_backend_gstreamer: fix build with gstreamer-1.6.1

An include flag which should be picked up through pkgconfig is not. The
root cause is unknown, but it's simple to add the missing flag to
NIX_CFLAGS_COMPILE.

pkgs/development/libraries/phonon-backend-gstreamer/qt5/default.nix

@@ -17,6 +17,11 @@ stdenv.mkDerivation rec {
 
   buildInputs = with gst_all_1; [ gstreamer gst-plugins-base phonon qtbase ];
 
+  NIX_CFLAGS_COMPILE = [
+    # This flag should be picked up through pkgconfig, but it isn't.
+    "-I${gst_all_1.gstreamer}/lib/gstreamer-1.0/include"
+  ];
+
   nativeBuildInputs = [ cmake pkgconfig ];
 
   cmakeFlags = [