package/s6-networking: fix dependency when libressl is enabled

Commit "c5b85231 s6-networking: enable SSL if libressl is selected"
actually dropped the dependency on s6-dns and s6 when libressl is
enabled.
Fix this by using += inside the conditional code.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>