build(thirdparty): make RocksDB depend on Hadoop before building HDFS plugin

[empiredan]

Add an explicit thirdparty build dependency from RocksDB to Hadoop.

RocksDB builds the HDFS plugin on Linux, and that plugin includes hdfs.h from the
Hadoop native client output. Without a CMake ExternalProject dependency, RocksDB
and Hadoop can build in parallel, causing RocksDB to compile before Hadoop has copied
hdfs.h into thirdparty/output/include/hdfs.

This fixes intermittent thirdparty Docker build failures such as:
fatal error: hdfs.h: No such file or directory

Validation:

• Verified the failure is caused by RocksDB HDFS plugin compiling before Hadoop native client output is available.
• The change serializes RocksDB after Hadoop while preserving existing thirdparty build behavior.