Development/Shake/Rules/Directory.hs: enable FlexibleContexts in implicit type sigs

> [36 of 41] Compiling Development.Shake.Rules.Directory ( Development/Shake/Rules/Directory.hs, dist/dist-sandbox-48f3b19e/build/Development/Shake/Rules/Directory.o )
>
> Development/Shake/Rules/Directory.hs:205:1:
>     Non type-variable argument
>       in the constraint: Rule key GetDirectoryA
>     (Use FlexibleContexts to permit this)
>     In the context: (Rule key GetDirectoryA)
>     While checking the inferred type for ‘getDirAction’

Fixes build failure on current ghc-HEAD due to:
http://permalink.gmane.org/gmane.comp.lang.haskell.glasgow.user/24612

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