buildroot

package/cpio: add argp-standalone dependency with musl

Since argp-standalone is only available for uClibc-ng
and musl toolchains, add the dependendy only if it's
selected.

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