libbpf: fixed build error for samples/bpf/

Commit 8a138aed4a ("bpf: btf: Add BTF support to libbpf") did not
include stdbool.h, so GCC complained when building samples/bpf/.

In file included from /home/btopel/src/ext/linux/samples/bpf/libbpf.h:6:0,
                 from /home/btopel/src/ext/linux/samples/bpf/test_lru_dist.c:24:
/home/btopel/src/ext/linux/tools/lib/bpf/bpf.h:105:4: error: unknown type name ‘bool’; did you mean ‘_Bool’?
    bool do_log);
    ^~~~
    _Bool

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/bpf.h

@@ -24,6 +24,7 @@
 #define __BPF_BPF_H
 
 #include <linux/bpf.h>
+#include <stdbool.h>
 #include <stddef.h>
 
 struct bpf_create_map_attr {