dev-lang/idris: Add missing dependencies, fixes Github issue #304.

According to http://www.idris-lang.org/documentation/faq/ and some
quick testing, dev-libs/gmp is required unconditionally. With the
default USE=llvm, we also need dev-libs/boehm-gc. Additionally,
the dep on dev-haskell/libffi was moved to RDEPEND since
/usr/bin/idris dynamically links to it.

Package-Manager: portage-2.2.7