Use pkgconfig-depends for fuse dependency

Fixes build failure on OS X; details in the commit comment.

I'm pretty new to Haskell so there's a decent chance I'm doing something dumb with this, but the [Cabal docs](https://www.haskell.org/cabal/release/cabal-latest/doc/users-guide/developing-packages.html#package-properties) say pkgconfig-depends is preferable so I ran with it.

I'm a little nervous about the impact of this on other platforms (Linux, FreeBSD, OS X with MacFUSE instead of OSXFUSE), since I haven't tested these. In theory, all of those should have their FUSE registered with pkg-config, so it *should* be fine... but testing this on another machine or two would probably be a good idea.