lfb-event: Fix SPDX identifier

The library is LGPL-2.1+, not GPL-3.0+

libfeedback/lfb-event.c

@@ -1,6 +1,6 @@
 /*
  * Copyright (C) 2020 Purism SPC
- * SPDX-License-Identifier: GPL-3.0+
+ * SPDX-License-Identifier: LGPL-2.1+
  * Author: Guido Günther <agx@sigxcpu.org>
  */
 

libfeedback/lfb-event.h

@@ -1,7 +1,7 @@
 /*
  * Copyright (C) 2020 Purism SPC
  *
- * SPDX-License-Identifier: GPL-3.0+
+ * SPDX-License-Identifier: LGPL-2.1+
  */
 #pragma once