Use correct printf formatter for size_t
Use correct printf formatter for size_t

The S3VectorUInt8 .size() function returns size_t, which is platform
dependent. Use the C99 %zu as the printf formatter since it's intended
to ensure the right format for each platform. This resolves the clang
compiler warning on this file:

src/s3key_writer.cpp:93:67: warning: format specifies type 'unsigned long long' but the
	  argument has type 'std::__1::vector<unsigned char, PGAllocator<unsigned char> >::size_type'
      (aka 'unsigned long') [-Wformat]
                (uint64_t) pthread_self(), params->currentNumber, params->data.size());
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

Discussion: https://github.com/greenplum-db/gpdb/pull/6396

50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb