PixelModuleDetector: Add license identifier

src/core/detector/PixelModuleDetector.cpp

@@ -4,6 +4,7 @@
  * This software is distributed under the terms of the MIT License, copied verbatim in the file "LICENSE.md".
  * In applying this license, CERN does not waive the privileges and immunities granted to it by virtue of its status as an
  * Intergovernmental Organization or submit itself to any jurisdiction.
+ * SPDX-License-Identifier: MIT
  */
 
 #include "PixelModuleDetector.hpp"