ruby: fix DEPENDS append

recent change create a blank scope of DEPENDS for class-target,
basically leaving out all general dependencies, leading to the effect
that ruby will be shipped without the runtime dependencies of zlib,
openssl and libffi, making the corresponding gems unusable at runtime.

As the class-target scope should be appended only the correct override
is append:class-target

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>