[Java] Correct the library loading for zlib in RocksJava.

Summary:
Correct the library loading for zlib in RocksJava: zlib should
be loaded by loadLibrary("z") instead of loadLibrary("zlib").

Test Plan:
make rocksdbjava
cd java
make db_bench
./jdb_bench.sh --compression_type=zlib

Reviewers: sdong, ljin, ankgup87

Reviewed By: ankgup87

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19341