package/lftp: don't use host's path

lftp try to link with -L/usr/lib/lftp/$(LFTP_VERSION) if
DESTDIR is not set.

Remove useless -L option which point to a location where
no libraries are installed.

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