wrk: build with debug information

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
wrk: fix uninitialized connection object

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>