Fixed clang build in env.h

Summary: Mark function as override.

Test Plan: USE_CLANG=1 make -j32 check

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D35163

include/rocksdb/env.h

@@ -814,7 +814,7 @@ class EnvWrapper : public Env {
     return target_->Schedule(f, a, pri, tag);
   }
 
-  int UnSchedule(void* tag, Priority pri) {
+  int UnSchedule(void* tag, Priority pri) override {
     return target_->UnSchedule(tag, pri);
   }