subdir is added to the OUTPUT which fails as part of building
install_headers when passed from "make -C tools perf_install".

Committer testing:

The original reporter (see the Link: below) had trouble with this:

$ make -C tools perf_install

That ended up with errors like this:

  /var/home/acme/git/perf-urgent/tools/scripts/Makefile.include:17: *** output directory "/var/home/acme/git/perf-urgent/tools/perf/libperf/perf/" does not exist.  Stop.

With this patch applied we now get it installed at:

  INSTALL /var/home/acme/git/perf-urgent/tools/perf/libperf/include/perf/bpf_perf.h

As expected:

  $ ls -la /var/home/acme/git/perf-urgent/tools/perf/libperf/include/perf/bpf_perf.h
  -rw-r--r--. 1 acme acme 1146 Jan  3 15:42 /var/home/acme/git/perf-urgent/tools/perf/libperf/include/perf/bpf_perf.h

And if we clean tools with:

  $ make -C tools clean

it gets cleaned up:

  $ ls -la /var/home/acme/git/perf-urgent/tools/perf/libperf/include/perf/bpf_perf.h
  ls: cannot access '/var/home/acme/git/perf-urgent/tools/perf/libperf/include/perf/bpf_perf.h': No such file or directory
  $

Fixes: 746bd29e ("perf build: Use tools/lib headers from install path")
Reported-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/fa4b3115-d555-3d7f-54d1-018002e99350@secunet.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>