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 <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/.gitignore

···
 .config-detected
 util/intel-pt-decoder/inat-tables.c
 arch/*/include/generated/
+pmu-events/pmu-events.c
+pmu-events/jevents