rpc: replacing ssh_get_publickey() by ssh_get_server_publickey().

After version 0.7.5, libssh deprecated the function scope
ssh_get_publickey() and moved to ssh_get_server_publickey(). So, Libvirt
is failing to compile using this new function name.
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

src/rpc/virnetlibsshsession.c

@@ -214,7 +214,7 @@ virLibsshServerKeyAsString(virNetLibsshSessionPtr sess)
     size_t keyhashlen;
     char *str;
 
-    if (ssh_get_publickey(sess->session, &key) != SSH_OK) {
+    if (ssh_get_server_publickey(sess->session, &key) != SSH_OK) {
         virReportError(VIR_ERR_LIBSSH, "%s",
                        _("failed to get the key of the current "
                          "session"));