[ Plot ]: Update SPDX License Identifiers

src/Mod/Plot/CMakeLists.txt

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
 SET(PlotMain_SRCS
     Plot.py
 )

src/Mod/Plot/Plot.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
 # ***************************************************************************
 # *   Copyright (c) 2011, 2012 Jose Luis Cercos Pita <[email protected]>   *
 # *                                                                         *