build: avoid build failure without gnutls

Found while trying to cross-compile to mingw:

  CC       libvirt_driver_remote_la-remote_driver.lo
../../src/remote/remote_driver.c: In function 'doRemoteOpen':
../../src/remote/remote_driver.c:487:23: error: variable 'verify' set but not used [-Werror=unused-but-set-variable]

* src/remote/remote_driver.c (doRemoteOpen): Also ignore 'verify'.
Signed-off-by: NEric Blake <eblake@redhat.com>
(cherry picked from commit 4e6a78e7)

src/remote/remote_driver.c

@@ -609,6 +609,7 @@ doRemoteOpen(virConnectPtr conn,
         priv->is_secure = 1;
 #else
         (void)sanity;
+        (void)verify;
         virReportError(VIR_ERR_INVALID_ARG, "%s",
                        _("GNUTLS support not available in this build"));
         goto failed;