Fix compile error on platforms without fallocate()
Fix compile error on platforms without fallocate()

Summary:
If a platform doesn't have ROCKSDB_FALLOCATE_PRESENT, then compiler complains:

util/env_posix.cc:354:8: error: private field 'allow_fallocate_' is not used [-Werror,-Wunused-private-field]

This was caught by travis.

Test Plan: compiles with ROCKSDB_FALLOCATE_PRESENT.

Reviewers: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D48327

facebook/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,Vaisman/rocksdb,facebook/rocksdb,SunguckLee/RocksDB,hobinyoon/rocksdb,facebook/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,bbiao/rocksdb,norton/rocksdb,flabby/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,Vaisman/rocksdb,hobinyoon/rocksdb,jalexanderqed/rocksdb,wenduo/rocksdb,norton/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,facebook/rocksdb,bbiao/rocksdb,Vaisman/rocksdb,Andymic/rocksdb,vmx/rocksdb,norton/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,wat-ze-hex/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,tsheasha/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,tsheasha/rocksdb,SunguckLee/RocksDB,Vaisman/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,ryneli/rocksdb,vmx/rocksdb,Andymic/rocksdb,wenduo/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,wenduo/rocksdb,Vaisman/rocksdb,bbiao/rocksdb,Vaisman/rocksdb,wat-ze-hex/rocksdb,flabby/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,ryneli/rocksdb,Andymic/rocksdb,OverlordQ/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,OverlordQ/rocksdb,flabby/rocksdb,hobinyoon/rocksdb,norton/rocksdb,jalexanderqed/rocksdb,OverlordQ/rocksdb,jalexanderqed/rocksdb,hobinyoon/rocksdb,norton/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wenduo/rocksdb,ryneli/rocksdb,wenduo/rocksdb,norton/rocksdb,OverlordQ/rocksdb,ryneli/rocksdb,wat-ze-hex/rocksdb,Vaisman/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,ryneli/rocksdb,vmx/rocksdb,facebook/rocksdb,norton/rocksdb,bbiao/rocksdb,bbiao/rocksdb,facebook/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,norton/rocksdb,jalexanderqed/rocksdb,facebook/rocksdb,Andymic/rocksdb,vmx/rocksdb,ryneli/rocksdb,bbiao/rocksdb,ryneli/rocksdb,tsheasha/rocksdb