libssh2: add recipe file

A separate commit on curl adds a PACKAGECONFIG option within the
curl recipe in OE-Core to use libssh2 in curl, the user can enable
libssh2 via conf/local.conf or custom distro configuration, this will
pull in libssh2, which is not used by default.

Signed-off-by: Fabrice Coulon <fabrice.coulon@axis.com>
Signed-off-by: Olof Johansson <olof.johansson@axis.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>