syscall: mksyscall_windows.go: put path separator in suffix for matching GOROOT

fixes #16493

Change-Id: I86bec2f9bd7965449c43e94733791f7cb18c5c4c
Reviewed-on: https://go-review.googlesource.com/25165Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/syscall/mksyscall_windows.go

@@ -708,6 +708,10 @@ func (src *Source) IsStdRepo() (bool, error) {
 		abspath = strings.ToLower(abspath)
 		goroot = strings.ToLower(goroot)
 	}
+	sep := string(os.PathSeparator)
+	if !strings.HasSuffix(goroot, sep) {
+		goroot += sep
+	}
 	return strings.HasPrefix(abspath, goroot), nil
 }