GStreamerSharp: improve previous commit to fix the build (bgo#724752)

Previous commit [1] fixed the build for MonoDevelop in Linux, but
could likely break the build for Windows, where we don't use pkg-config
(we just place the dependencies in a directory). So seems like using
<HintPath> along with <Package> element doesn't generate any conflicts.

[1] https://git.gnome.org/browse/banshee/commit/?id=1ea5969f81f951009202e4884b01eda6ffd671f7