perf tools: Make usage of test_attr__* optional for perf-sys.h

For users of perf-sys.h outside perf, e.g. samples/bpf/bpf_load.c, it's
convenient not to depend on test_attr__*.

After commit 91854f9a ("perf tools: Move everything related to
sys_perf_event_open() to perf-sys.h"), all users of perf-sys.h will
depend on test_attr__enabled and test_attr__open.

This commit enables a user to define HAVE_ATTR_TEST to zero in order
to omit the test dependency.

Fixes: 91854f9a ("perf tools: Move everything related to sys_perf_event_open() to perf-sys.h")
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Acked-by: Song Liu <songliubraving@fb.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: bpf@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: http://lore.kernel.org/bpf/20191001113307.27796-2-bjorn.topel@gmail.com