Fixed a compile warning in options_test.cc under clang

Summary:
Fixed the following compile warning in options_test.cc under clang
util/options_test.cc:94:12: error: 'Skip' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
    Status Skip(uint64_t n) {
           ^
./include/rocksdb/env.h:368:18: note: overridden virtual function is here
  virtual Status Skip(uint64_t n) = 0;
                 ^

Test Plan: options_test

Reviewers: igor, sdong, anthony, IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47763

util/options_test.cc

@@ -91,7 +91,7 @@ class StringEnv : public EnvWrapper {
       }
       return Status::OK();
     }
-    Status Skip(uint64_t n) {
+    Status Skip(uint64_t n) override {
       if (offset_ >= data_.size()) {
         return Status::InvalidArgument(
             "Attemp to read when it already reached eof.");