windows: add LoadGetSystemTimePreciseAsFileTime

Useful since GetSystemTimePreciseAsFileTime is Windows 8+.

Change-Id: I736bd0668aaf6ad086f6465b8b9a17f1943f0570
Reviewed-on: https://go-review.googlesource.com/43312
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/syscall_windows.go

@@ -486,6 +486,10 @@ func Chmod(path string, mode uint32) (err error) {
 	return SetFileAttributes(p, attrs)
 }
 
+func LoadGetSystemTimePreciseAsFileTime() error {
+	return procGetSystemTimePreciseAsFileTime.Find()
+}
+
 func LoadCancelIoEx() error {
 	return procCancelIoEx.Find()
 }