x86/smp: Export symbol cpu_clustergroup_mask()

Intel cstate PMU driver will invoke the topology_cluster_cpumask() to
retrieve the CPU mask of a cluster. A modpost error is triggered since
the symbol cpu_clustergroup_mask is not exported.
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20231116142245.1233485-2-kan.liang@linux.intel.com

arch/x86/kernel/smpboot.c

@@ -757,6 +757,7 @@ const struct cpumask *cpu_clustergroup_mask(int cpu)
 {
 	return cpu_l2c_shared_mask(cpu);
 }
+EXPORT_SYMBOL_GPL(cpu_clustergroup_mask);
 
 static void impress_friends(void)
 {