libbpf: add missing SPDX-License-Identifier

Signed-off-by: Eric Leblond <eric@regit.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/bpf.c

+// SPDX-License-Identifier: LGPL-2.1
+
 /*
  * common eBPF ELF operations.
  *

tools/lib/bpf/bpf.h

+/* SPDX-License-Identifier: LGPL-2.1 */
+
 /*
  * common eBPF ELF operations.
  *

tools/lib/bpf/libbpf.c

+// SPDX-License-Identifier: LGPL-2.1
+
 /*
  * Common eBPF ELF object loading operations.
  *

tools/lib/bpf/libbpf.h

+/* SPDX-License-Identifier: LGPL-2.1 */
+
 /*
  * Common eBPF ELF object loading operations.
  *