Fix clang build

Summary: Fix clang

Test Plan: make check

Reviewers: sdong, yhchiang, rven

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D51417

include/rocksdb/env.h

@@ -769,9 +769,9 @@ class EnvWrapper : public Env {
     return target_->NewWritableFile(f, r, options);
   }
   Status ReuseWritableFile(const std::string& fname,
-			   const std::string& old_fname,
-			   unique_ptr<WritableFile>* r,
-			   const EnvOptions& options) {
+                           const std::string& old_fname,
+                           unique_ptr<WritableFile>* r,
+                           const EnvOptions& options) override {
     return target_->ReuseWritableFile(fname, old_fname, r, options);
   }
   virtual Status NewDirectory(const std::string& name,