perf/x86/intel: Check the X86 leader for pebs_counter_event_group

The PEBS counters snapshotting group also requires a group flag in the
leader. The leader must be a X86 event.

Fixes: e02e9b03 ("perf/x86/intel: Support PEBS counters snapshotting")
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250424134718.311934-3-kan.liang@linux.intel.com