gnu: git-annex: Update to 8.20200908.

* gnu/packages/haskell-apps.scm (git-annex): Update to 8.20200908.
[arguments]: Drop flag to disable WebDAV because the latest release
unconditionally builds with WebDAV support.
[inputs]: Add ghc-dav.

Signed-off-by: Timothy Sample <samplet@ngyro.com>