package/lftp: inverse the openssl/gnutls selection

openssl is more widely used than gnutls. Other packages prefer openssl
over gnutls and would like to do the select in the opposite direction.

So switch lftp to use openssl by default, and only revert to using
gnutls if explicitly selected by the user.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>