perf: Update .gitignore file

After a "make -C tools/perf", git reports the following untracked file:
perf-iostat

Add this generated file to perf's .gitignore file.
Signed-off-by: Alexander Antonov <alexander.antonov@linux.intel.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey V Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210419094147.15909-5-alexander.antonov@linux.intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/.gitignore

@@ -20,6 +20,7 @@ perf.data.old
 output.svg
 perf-archive
 perf-with-kcore
+perf-iostat
 tags
 TAGS
 cscope*