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