Make it possible to make Haskell bindings for C libraries that depend on freetype

[Peaker]

Add the install-includes needed so that one can make bindings for C libraries that depend on freetype. Otherwise, to make these bindings you have to copy the entire freetype2 C headers into your own bindings' package.

In particular, I am making bindings for freetype-gl so I can use the library with OpenGL portably. (FTGL doesn't build on Windows, at least not easily).