perf tools: Add the legacy-cache.json to .gitignore

The commit 0012e0fa221bf9cc ("perf jevents: Add legacy-hardware and
legacy-cache json") will auto-generate: "pmu-events/arch/common/common/legacy-cache.json".

Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Haiyue Wang <haiyuewa@163.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.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>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>