lib/percpu_counter.c: enclose hotplug only variables in hotplug ifdef

These variables are only used when CONFIG_HOTPLUG_CPU is enabled, they are
ifdef'ed everywhere else.  So don't define them when CONFIG_HOTPLUG_CPU is
not enabled.
Signed-off-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/percpu_counter.c

@@ -10,8 +10,10 @@
 #include <linux/module.h>
 #include <linux/debugobjects.h>
 
+#ifdef CONFIG_HOTPLUG_CPU
 static LIST_HEAD(percpu_counters);
 static DEFINE_MUTEX(percpu_counters_lock);
+#endif
 
 #ifdef CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER