perf bench bpf: Add missing .gitignore file

perf bench bpf: Add missing .gitignore file

In 713eeb2279402758 ("perf build: Move BPF skeleton generation out of
Makefile.perf") the bpf_skel used with 'perf bench uprobe' was moved
from tools/perf/util/bpf_skel/ to tools/perf/bench/bpf_skel.

Copy tools/perf/util/bpf_skel/.gitignore to that new directory so that
files generated during build get ignored by git.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Fixes: 713eeb2279402758 ("perf build: Move BPF skeleton generation out of Makefile.perf")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>