bpftool: Add SPDX identifier to btf-dump-file output

A concern about potential GPL violations came up at the new $DAYJOB when
I tried to vendor the vmlinux.h output. The central point was that the
generated vmlinux.h does not embed a license string -- making the
licensing of the file non-obvious.

This commit adds a LGPL-2.1 OR BSD-2-Clause SPDX license identifier to
the generated vmlinux.h output. This is line with what bpftool generates
in object file skeletons.

Signed-off-by: Daniel Xu <[email protected]>

Author: danobi

tools/bpf/bpftool/btf.c

@@ -425,6 +425,7 @@ static int dump_btf_c(const struct btf *btf,
 	if (err)
 		return err;
 
+	printf("/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */\n\n");
 	printf("#ifndef __VMLINUX_H__\n");
 	printf("#define __VMLINUX_H__\n");
 	printf("\n");