git-annex: add missing USE=doc (found by icedev)

 * ERROR: dev-vcs/git-annex-4.20130627-r1::gentoo-haskell failed (compile phase):
  *   USE Flag 'doc' not in IUSE for dev-vcs/git-annex-4.20130627-r1

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>