sys-apps/iproute2: fix automagic libelf dep.

sys-apps/iproute2: fix automagic libelf dep.

iproute2 has been automagically linking against libelf for a long time.
It is only used for eBPF programs, so isn't critical in most cases.

Declare the dep via virtual/libelf, but put it behind USE=-minimal, as
with libmnl.

Closes: https://bugs.gentoo.org/638282
Package-Manager: Portage-2.3.8, Repoman-2.3.3
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>