Add missing 'explicit's on single-argument constructors.

Change-Id: I1494df6e74ec16238971fb4346ba184eb61c37ab
diff --git a/src/oat_file.h b/src/oat_file.h
index 1d3367c..e83b294 100644
--- a/src/oat_file.h
+++ b/src/oat_file.h
@@ -126,7 +126,7 @@
   }
 
  private:
-  OatFile(const std::string& filename);
+  explicit OatFile(const std::string& filename);
   bool Read(const std::string& filename, byte* requested_base);
 
   const byte* GetBase() const;