Update .gitignore file in examples

Summary:
options_file_example should be added in .gitignore so that it does not show up as an untracked file in `git status`.
Closes https://github.com/facebook/rocksdb/pull/2026

Differential Revision: D4759402

Pulled By: sagar0

fbshipit-source-id: d7fe133

examples/.gitignore

@@ -3,5 +3,6 @@ column_families_example
 compact_files_example
 compaction_filter_example
 optimistic_transaction_example
+options_file_example
 simple_example
 transaction_example