tools: bpftool: fix SPDX format in headers

Documentation/process/license-rules.rst sayeth:

2. Style:

The SPDX license identifier is added in form of a comment.  The comment
style depends on the file type::

   C source: // SPDX-License-Identifier: <SPDX License Expression>
   C header: /* SPDX-License-Identifier: <SPDX License Expression> */

Headers should use C comment style.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: NRoman Gushchin <guro@fb.com>
Acked-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NYonghong Song <yhs@fb.com>
Acked-by: NStanislav Fomichev <sdf@google.com>
Acked-by: NSean Young <sean@mess.org>
Acked-by: NJiri Benc <jbenc@redhat.com>
Acked-by: NDavid Calavera <david.calavera@gmail.com>
Acked-by: NAndrey Ignatov <rdna@fb.com>
Acked-by: NJoe Stringer <joe@wand.net.nz>
Acked-by: NDavid Ahern <dsahern@gmail.com>
Acked-by: NAlexei Starovoitov <alexei.starovoitov@gmail.com>
Acked-by: NPetar Penkov <ppenkov@stanford.edu>
Acked-by: NSandipan Das <sandipan@linux.ibm.com>
Acked-by: NPrashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: NStephen Hemminger <stephen@networkplumber.org>
Acked-by: NJohn Fastabend <john.fastabend@gmail.com>
Acked-by: NTaeung Song <treeze.taeung@gmail.com>
Acked-by: NJiri Olsa <jolsa@kernel.org>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
CC: okash.khawaja@gmail.com
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/cfg.h

-// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+/* SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) */
 /*
  * Copyright (C) 2018 Netronome Systems, Inc.
  *

tools/bpf/bpftool/netlink_dumper.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 // Copyright (C) 2018 Facebook
 
 #ifndef _NETLINK_DUMPER_H_

tools/bpf/bpftool/xlated_dumper.h

-// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+/* SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) */
 /*
  * Copyright (C) 2018 Netronome Systems, Inc.
  *