windows: add SetFileValidData

Fixes golang/go#61834

Change-Id: Iaff01f5662aa2309e8ac41b346e0d3a42d28af91
Reviewed-on: https://go-review.googlesource.com/c/sys/+/517575
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>

windows/syscall_windows.go

@@ -194,6 +194,7 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	GetComputerName(buf *uint16, n *uint32) (err error) = GetComputerNameW
 //sys	GetComputerNameEx(nametype uint32, buf *uint16, n *uint32) (err error) = GetComputerNameExW
 //sys	SetEndOfFile(handle Handle) (err error)
+//sys	SetFileValidData(handle Handle, validDataLength int64) (err error)
 //sys	GetSystemTimeAsFileTime(time *Filetime)
 //sys	GetSystemTimePreciseAsFileTime(time *Filetime)
 //sys	GetTimeZoneInformation(tzi *Timezoneinformation) (rc uint32, err error) [failretval==0xffffffff]