Cleanup: Add missing license and copyright headers

This PR adds the standard license header that was missed when the file was originally created.

Pull Request: https://projects.blender.org/blender/blender/pulls/119105

source/blender/editors/sculpt_paint/sculpt_gesture.cc

@@ -1,3 +1,7 @@
+/* SPDX-FileCopyrightText: 2024 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
 /** \file
  * \ingroup edsculpt
  * Common helper methods and structures for gesture operations.