package/rng-tools: needs argp-standalone with musl toolchains

Since argp-standalone is only available for uClibc-ng
and musl toolchains, it's safe to link with it when
the package is selected.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>