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.