keep util/build_version.cc when make clean
keep util/build_version.cc when make clean

Summary:
https://github.com/facebook/rocksdb/pull/2264
adding build_version.cc into clean list which breaks fbcode release.
we need to keep it when `make clean`
Closes https://github.com/facebook/rocksdb/pull/2322

Differential Revision: D5088932

Pulled By: lightmark

fbshipit-source-id: ab001424af596e94a6bc1d4186c39edf6ace484f

SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb