write_hist_log: do not require ZLIB for non-server instances

Since `td->client_type` is always non-zero (CLI is 1, GUI is 2), the check
`if (td->client_type)` in `init.c` always triggers, causing --write_hist_log
to be unusable when ZLIB is not configured for standalone runs.
By checking if the fio instance `is_backend`, ZLIB availability will be
checked for fio running as server in server/client mode, and the check
will be passed for fio running in standalone mode.
1 file changed