[REEF-1884] Fixed a missing `import` in `IFailureStateMachine.cs`

JIRA:
  [REEF-1884](https://issues.apache.org/jira/browse/REEF-1884)

Pull Request:
  This closes #1376
diff --git a/lang/cs/Org.Apache.REEF.Network/Elastic/Failures/IFailureStateMachine.cs b/lang/cs/Org.Apache.REEF.Network/Elastic/Failures/IFailureStateMachine.cs
index e5a3b44..3126f63 100644
--- a/lang/cs/Org.Apache.REEF.Network/Elastic/Failures/IFailureStateMachine.cs
+++ b/lang/cs/Org.Apache.REEF.Network/Elastic/Failures/IFailureStateMachine.cs
@@ -16,6 +16,7 @@
 // under the License.
 
 using System;
+using Org.Apache.REEF.Utilities.Attributes;
 
 namespace Org.Apache.REEF.Network.Elastic.Failures
 {