perf tools: Add the empty-pmu-events build to .gitignore

The commit 0fe881f10ceb ("perf jevents: Autogenerate empty-pmu-events.c")
build will generate two files, add them to .gitignore:

tools/perf/pmu-events/empty-pmu-events.log
tools/perf/pmu-events/test-empty-pmu-events.c

Signed-off-by: Haiyue Wang <[e-mail]>
Reviewed-by: Ian Rogers <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/.gitignore

···
 pmu-events/pmu-events.c
 pmu-events/jevents
 pmu-events/metric_test.log
+pmu-events/empty-pmu-events.log
+pmu-events/test-empty-pmu-events.c
 *.shellcheck_log
 feature/
 libapi/