Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/file_output_stream.h b/runtime/file_output_stream.h
index 10405ef..23a57f5 100644
--- a/runtime/file_output_stream.h
+++ b/runtime/file_output_stream.h
@@ -25,7 +25,7 @@
 
 class FileOutputStream : public OutputStream {
  public:
-  FileOutputStream(File* file);
+  explicit FileOutputStream(File* file);
 
   virtual ~FileOutputStream() {}