package/systemd: needs glibc

Since version v239, systemd-nspawn unconditioanlly uses prlimit(2),
which is not implemented in uClibc-ng. systemd-nspawn can not be
disabled.

This makes systemd glibc-only again.

After a bit of discussion with upstream (om IRC), it looks very
improbable that they accept a patch making systemd-nspawn optional.
They would probably consider a patch that provides that syscall wrapper
if it is missing, though, but that's less trivial...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>