_ptw32.h not found in sched.h on VS2015 builds

[g4jc]

I'm not sure if this is a bug in the VS2015 project file or if there's been a typo for 6 years in sched.h, but the following fixes the build error for me and I was able to get it to build.

Change line 47 from: #include <_ptw32.h>

to

#include "_ptw32.h"

[GerHobbelt]

I'll treat it as an odd one-off bug in VS2015 (which I don't have installed any more) as none of the other compilers (including VS2019) complain about this AFAICT.

Keeping it open in case anyone else stumbles across this issue.

Thanks for reporting!