perf/x86/intel: Fix unintended variable name reuse

The variable name events_group is already in used and led to a
compilation error when using clang to build the Linux Kernel .
The fix is just to rename the var. No functional change. Please
apply.

Fix suggested in discussion by PaX Team <pageexec@freemail.hu>

Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
Cc: rostedt@goodmis.org
Cc: a.p.zijlstra@chello.nl
Cc: paulus@samba.org
Cc: acme@ghostprotocols.net
Link: http://lkml.kernel.org/r/1367316153-14808-1-git-send-email-dl9pf@gmx.de
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>