Fix includes for env_test
Fix includes for env_test

Summary:
Previously compilation failed when ROCKSDB_NO_FBCODE=1 because fcntl.h
wasn't included for open().

Related issue: https://github.com/facebook/rocksdb/issues/977

Test Plan:
verified below command works now:

  $ make clean && ROCKSDB_NO_FBCODE=1 ROCKSDB_DISABLE_FALLOCATE=1 make -j32 env_test

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54135

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