Fix missing imports

Author: shiling

src/main/java/picoded/dstack/mongodb/MongoDB_FileWorkspaceMap.java

@@ -16,6 +16,7 @@
 import java.sql.Date;
 
 // JavaCommons imports
+import com.mongodb.client.gridfs.model.GridFSDownloadOptions;
 import picoded.core.common.EmptyArray;
 import picoded.core.file.FileUtil;
 import picoded.dstack.FileWorkspace;