rkt: Fix incomplete selinux context string when the option is partial.

Add Getfilecon() into the selinux interface.

pkg/util/selinux/selinux_unsupported.go

@@ -18,9 +18,14 @@ limitations under the License.
 
 package selinux
 
-type realChconRunner struct{}
+type realSelinuxContextRunner struct{}
 
-func (_ *realChconRunner) SetContext(dir, context string) error {
+func (_ *realSelinuxContextRunner) SetContext(dir, context string) error {
 	// NOP
 	return nil
 }
+
+func (_ *realSelinuxContextRunner) Getfilecon(path string) (string, error) {
+	// NOP
+	return "", nil
+}