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 <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Song Liu <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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