bpf: btf: Change tools/lib/bpf/btf to LGPL

This patch changes the tools/lib/bpf/btf.[ch] to LGPL which
is inline with libbpf also.
Signed-off-by: NMartin KaFai Lau <kafai@fb.com>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/btf.c

-/* SPDX-License-Identifier: GPL-2.0 */
+// SPDX-License-Identifier: LGPL-2.1
 /* Copyright (c) 2018 Facebook */
 
 #include <stdlib.h>

tools/lib/bpf/btf.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: LGPL-2.1 */
 /* Copyright (c) 2018 Facebook */
 
 #ifndef __BPF_BTF_H