perf tools: Ignore generated files pmu-events/{jevents,pmu-events.c} for git

Ignore two files: pmu-events/{jevents,pmu-events.c} which are generated
during the build.

Committer notes:

Testing it:

  $ make -C tools/perf/
  $ git status
  On branch perf/core
  Untracked files:
  (use "git add <file>..." to include in what will be committed)

	tools/perf/pmu-events/jevents
	tools/perf/pmu-events/pmu-events.c

  nothing added to commit but untracked files present (use "git add" to track)
  $

After the patch:

  $ git status
  On branch perf/core
  nothing to commit, working tree clean
  $
Signed-off-by: Changbin Du <changbin.du@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170313083026.23487-1-changbin.du@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/.gitignore

@@ -31,3 +31,5 @@ config.mak.autogen
 .config-detected
 util/intel-pt-decoder/inat-tables.c
 arch/*/include/generated/
+pmu-events/pmu-events.c
+pmu-events/jevents