sci-mathematics/agda-stdlib: Install agda-lib-ffi

The agda-lib-ffi package provides some additional haskell modules which
are needed by the agda standard library, such as during compilation
using the MAlonzo backend. It is available inside the ffi/ directory and
needs to be built and installed by the ebuild. This patch does that.
Merge pull request #278 from vikraman/agda-stdlib_ffi

sci-mathematics/agda-stdlib: Install agda-lib-ffi