Import auth to support different platforms

Antctl failed to talk with GCP cluster and it shows:
`Error: no Auth Provider found for name "gcp"`. According to
the example of client-go, we need to import auth package to
support different public platform's authentication.

Signed-off-by: Lan Luo <luola@vmware.com>

pkg/antctl/runtime/runtime.go

@@ -18,6 +18,7 @@ import (
 	"os"
 	"strings"
 
+	_ "k8s.io/client-go/plugin/pkg/client/auth"
 	"k8s.io/client-go/rest"
 	"k8s.io/client-go/tools/clientcmd"