HPCC-10837 ws_workunits build break if USE_ZLIB=false

When -DUSE_ZLIB=false is specified in CMAKE command line, ws_workunits
build fails because of references to data types defined in zcrypt.hpp.
This fix wraps those references in #ifdef _USE_ZLIB to protect them

Signed-off-by: William Whitehead <william.whitehead@lexisnexis.com>