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

Change-Id: I1494df6e74ec16238971fb4346ba184eb61c37ab
diff --git a/src/zip_archive.h b/src/zip_archive.h
index 13a22cc..acf18b0 100644
--- a/src/zip_archive.h
+++ b/src/zip_archive.h
@@ -113,7 +113,7 @@
   }
 
  private:
-  ZipArchive(int fd) : fd_(fd), num_entries_(0), dir_offset_(0) {}
+  explicit ZipArchive(int fd) : fd_(fd), num_entries_(0), dir_offset_(0) {}
 
   bool MapCentralDirectory();
   bool Parse();