sdk: Explicitly install requirements for debos uml backend

Debos has gained support for user-mode-linux as a backend; to run this backend in the sdk will require the installation of user-mode-linux and the libslirp networking packages. So install these packages to allow the sdk images to use the uml fakemachine backend when building debos recipes.

Signed-off-by: Christopher Obbard chris.obbard@collabora.com