phonon-backend-gstreamer: don't create $out/$out/share/icons

The build system tries to update the mtime of the icons directory if
`DESTDIR` is unset. That code has bug though that does not deal well
with absolute `CMAKE_SHARE_PREFIX` resulting a double prefix bug.

Setting `DESTDIR=/` (should be a no-op) fixes this.

pkgs/development/libraries/phonon/backends/gstreamer.nix

···
     "-I${gst_all_1.gstreamer.dev}/lib/gstreamer-1.0/include"
   ];
 
+  # cleanup: the build system creates (empty) $out/$out/share/icons (double prefix)
+  # if DESTDIR is unset
+  DESTDIR="/";
+
   nativeBuildInputs = [ cmake pkgconfig ] ++ optional withQt5 extra-cmake-modules;
 
   cmakeFlags =