fix import lines at MessagePackExample that made sbt failed

Author: frsyuki

msgpack-core/src/test/java/org/msgpack/core/example/MessagePackExample.java

@@ -15,9 +15,13 @@
 //
 package org.msgpack.core.example;
 
-import org.msgpack.core.*;
+import org.msgpack.core.MessagePack;
 import org.msgpack.core.MessagePack.PackerConfig;
 import org.msgpack.core.MessagePack.UnpackerConfig;
+import org.msgpack.core.MessageBufferPacker;
+import org.msgpack.core.MessageFormat;
+import org.msgpack.core.MessagePacker;
+import org.msgpack.core.MessageUnpacker;
 import org.msgpack.value.ArrayValue;
 import org.msgpack.value.ExtensionValue;
 import org.msgpack.value.FloatValue;