Add override for overridden function in zlib

Bug: 428099
Change-Id: I33a10720a3231dec560aa431dba182929226f664
Reviewed-on: https://chromium-review.googlesource.com/1036716Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555252}

third_party/zlib/google/zip.cc

@@ -71,7 +71,7 @@ class DirectFileAccessor : public FileAccessor {
   }
 
   std::vector<DirectoryContentEntry> ListDirectoryContent(
-      const base::FilePath& dir) {
+      const base::FilePath& dir) override {
     std::vector<DirectoryContentEntry> files;
     base::FileEnumerator file_enumerator(
         dir, false /* recursive */,