Fix build on FreeBSD
Fix build on FreeBSD

Summary:
```
  CC       utilities/column_aware_encoding_exp.o
utilities/column_aware_encoding_exp.cc:149:5: error: use of undeclared identifier 'exit'
    exit(1);
    ^
utilities/column_aware_encoding_exp.cc:154:5: error: use of undeclared identifier 'exit'
    exit(1);
    ^
utilities/column_aware_encoding_exp.cc:158:5: error: use of undeclared identifier 'exit'
    exit(1);
    ^
3 errors generated.
```
Closes https://github.com/facebook/rocksdb/pull/1754

Differential Revision: D4399044

Pulled By: IslamAbdelRahman

fbshipit-source-id: fbab5a2

Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,SunguckLee/RocksDB