download.mk: fix packed checkout mirroring support

Changeset r48416  broke the downloading of mirrored, packed scm checkouts.

Fix this by removing the "@" sign in front of the download command which is
now executed as part of a larger shell command under flock.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 48733