Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/output_stream.h b/runtime/output_stream.h
index d2a77d8..aff6bcd 100644
--- a/runtime/output_stream.h
+++ b/runtime/output_stream.h
@@ -33,7 +33,7 @@
 
 class OutputStream {
  public:
-  OutputStream(const std::string& location) : location_(location) {}
+  explicit OutputStream(const std::string& location) : location_(location) {}
 
   virtual ~OutputStream() {}