Fix GCC compile warning for "format-signedness" in Windows

* src/job.c (reap_children): Use unsigned int for DWORD.
* src/w32/subproc/sub_proc.c (process_easy): Ditto.
* src/w32/w32os.c (jobserver_setup): Ditto.
(jobserver_release): Ditto.
(jobserver_acquire): Ditto.
3 files changed