Add missing override keyword in env_win.h functions (#11232)

Summary:
I couldn't figure out why this causes failures in our 8.0 release to fbcode while this issue appears to not be new in 8.0. Anyways, we can add the missing `override` keywords to these functions as the compiler insists.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11232

Reviewed By: pdillinger

Differential Revision: D43420656

Pulled By: ajkr

fbshipit-source-id: da748eeef6ba38dd113dbe4b5143d7558daf38dd

port/win/env_win.h

@@ -97,7 +97,7 @@ class WinClock : public SystemClock {
 
   Status GetCurrentTime(int64_t* unix_time) override;
   // Converts seconds-since-Jan-01-1970 to a printable string
-  virtual std::string TimeToString(uint64_t time);
+  std::string TimeToString(uint64_t time) override;
 
   uint64_t GetPerfCounterFrequency() const { return perf_counter_frequency_; }
 
@@ -116,7 +116,7 @@ class WinFileSystem : public FileSystem {
   ~WinFileSystem() {}
   static const char* kClassName() { return "WinFS"; }
   const char* Name() const override { return kClassName(); }
-  const char* NickName() const { return kDefaultName(); }
+  const char* NickName() const override { return kDefaultName(); }
 
   static size_t GetSectorSize(const std::string& fname);
   size_t GetPageSize() const { return page_size_; }