perf build: Install libbpf headers locally when building

API headers from libbpf should not be accessed directly from the
library's source directory. Instead, they should be exported with "make
install_headers". Let's adjust perf's Makefile to install those headers
locally when building libbpf.

v2:

- Fix $(LIBBPF_OUTPUT) when $(OUTPUT) is null.
- Make sure the recipe for $(LIBBPF_OUTPUT) is not under a "ifdef".

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: bpf@vger.kernel.org
Cc: netdev@vger.kernel.org
Link: http://lore.kernel.org/lkml/20211107002445.4790-1-quentin@isovalent.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>