fix build failure

Author: kiszk

common/unsafe/src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java

@@ -29,6 +29,7 @@
 import org.apache.spark.unsafe.memory.ByteArrayMemoryBlock;
 import org.apache.spark.unsafe.memory.MemoryBlock;
 import org.apache.spark.unsafe.memory.OnHeapMemoryBlock;
+import org.junit.Ignore;
 import org.junit.Test;
 
 import static org.junit.Assert.*;