windows: allow re-enabling priority boost

The Go runtime disables priority boosting because it interferes with the
scheduler timer threads in some cases. But some apps might want to
re-enable it for specific reasons, so this commit adds the function to
do so.

Change-Id: Ida68b9f2b188560b46c322197461dd06453329e2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/191839
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/syscall_windows.go

@@ -269,6 +269,7 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	GenerateConsoleCtrlEvent(ctrlEvent uint32, processGroupID uint32) (err error)
 //sys	GetProcessId(process Handle) (id uint32, err error)
 //sys	OpenThread(desiredAccess uint32, inheritHandle bool, threadId uint32) (handle Handle, err error)
+//sys	SetProcessPriorityBoost(process Handle, disable bool) (err error) = kernel32.SetProcessPriorityBoost
 
 // Volume Management Functions
 //sys	DefineDosDevice(flags uint32, deviceName *uint16, targetPath *uint16) (err error) = DefineDosDeviceW