perf: Use consistent include paths for libbpf

Fix perf to include libbpf header files with the bpf/ prefix, to
be consistent with external users of the library.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/157952560797.1683545.7685921032671386301.stgit@toke.dk