libssh: update to just after 0.6.4

Among other things, includes a fix for CVE-2014-8132.

(There are a couple of useful-looking fixes after 0.6.4 in the 0.6
branch, hence I have renamed the recipe to _git and set PV to the
standard format.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>