perf test bpf: Free obj_buf

perf test bpf: Free obj_buf

ASan reports some memory leaks when running:

  # perf test "42: BPF filter"

The first of these leaks is caused by obj_buf never being deallocated in
__test__bpf.

This patch adds the missing free.

Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Fixes: ba1fae431e74bb42 ("perf test: Add 'perf test BPF'")
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lore.kernel.org/lkml/60f3ca935fe6672e7e866276ce6264c9e26e4c87.1626343282.git.rickyman7@gmail.com
[ Added missing stdlib.h include ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>