Remove it from the list of tools if --disable-system
and --disable-tools are used as we don't need it for
linux-user targets.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
[lv: I also disable it with disable-tools, not only with disable-system]
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20190401141222.30034-4-lvivier@redhat.com>