[libvirt] Build fails on Debian unstable: libssh's ssh_get_publickey() is deprecated

[Andrea Bolognani]

I ran into a failure trying to compile libvirt on Debian unstable
today:

  rpc/virnetlibsshsession.c: In function 'virLibsshServerKeyAsString':
  rpc/virnetlibsshsession.c:220:5: error: 'ssh_get_publickey' is deprecated [-Werror=deprecated-declarations]
       if (ssh_get_publickey(sess->session, &key) != SSH_OK) {
       ^~
  In file included from rpc/virnetlibsshsession.c:24:0:
  /usr/include/libssh/libssh.h:489:31: note: declared here
   SSH_DEPRECATED LIBSSH_API int ssh_get_publickey(ssh_session session, ssh_key *key);
                                 ^~~~~~~~~~~~~~~~~
    CC       libvirt_lxc_la-libvirt-lxc.lo
  cc1: all warnings being treated as errors
  Makefile:8092: recipe for target 'rpc/libvirt_net_rpc_la-virnetlibsshsession.lo' failed

Looks like Debian decided to start shipping a development snapshot
instead of the latest stable release:

  $ dpkg -l | grep libssh-
  ii  libssh-gcrypt-4:amd64    0.8.0~20170825.94fa1e38-1
  ii  libssh-gcrypt-dev        0.8.0~20170825.94fa1e38-1

That means we're not in a hurry; still, it probably makes sense to
try and figure out a way to keep it compiling (and working!) sooner
rather than later :)

-- 
Andrea Bolognani / Red Hat / Virtualization