bpf: update bpf.h uapi header for tools

Update tools/include/uapi/linux/bpf.h file in order to reflect the
changes for BPF helper functions documentation introduced in previous
commits.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>