license: relicense "libnm-core/nm-errors.h" under LGPL-2.1+

$ git shortlog -n -s a3e75f3294 -- libnm-core/nm-errors.h
      11    Dan Winship
       5    Thomas Haller
       2    Beniamino Galvani

Note that the header also contains code that was copied from other
files. However, that code originates from libnm itself (and thus was
already LGPL licensed).

All contributors agreed to the relicensing according to "RELICENSE.md".

libnm-core/nm-errors.h

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: LGPL-2.1+
 /*
  * Copyright (C) 2004 - 2014 Red Hat, Inc.
  */