all: unify comment style for SPDX-License-Identifier tag

Our coding style recommends C style comments (/* */) instead of C++
(//). Also, systemd (which we partly fork) uses C style comments for
the SPDX-License-Identifier.

Unify the style.

  $ sed -i '1 s#// SPDX-License-Identifier: \([^ ]\+\)$#/* SPDX-License-Identifier: \1 */#' -- $(git ls-files -- '*.[hc]' '*.[hc]pp')

examples/C/glib/add-connection-gdbus.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Copyright (C) 2011, 2014 Red Hat, Inc.
  */