perf tools: Add PERF-FEATURES to the .gitignore file

It's an auto-generated file.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <david.ahern@oracle.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150228081248.GA31856@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>