libssh2: upgrade to 1.7.0

This version includes fix for CVE-2016-0787
and other changes, for details please see:
https://www.libssh2.org/changes.html



Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
(cherry picked from commit df5141af8dfd19c2b2dbfb65f58eda7a509fafd8)
Signed-off-by: Armin Kuster <akuster808@gmail.com>