lib/osutil: Return "/" as filesystem root on non-windows (fixes #3321)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3332

lib/osutil/fsroots_unix.go

@@ -0,0 +1,13 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// +build !windows
+
+package osutil
+
+func GetFilesystemRoots() ([]string, error) {
+	return []string{"/"}, nil
+}