BannedFilesTest: fix build.

I wrote my code in master, where this include is already
present. I didn't test the code when I backported it to jb-dev.
*sigh*

Change-Id: Ief8f7b1e98e58ef75fd9876d0f7ab8fbe1a72595
diff --git a/tests/tests/security/src/android/security/cts/BannedFilesTest.java b/tests/tests/security/src/android/security/cts/BannedFilesTest.java
index 132d8cb..6913fb2 100644
--- a/tests/tests/security/src/android/security/cts/BannedFilesTest.java
+++ b/tests/tests/security/src/android/security/cts/BannedFilesTest.java
@@ -20,6 +20,8 @@
 
 import junit.framework.TestCase;
 
+import java.io.File;
+
 public class BannedFilesTest extends TestCase {
 
     public void testNoSyncAgent() {