drivers/cpufreq/cpufreq.c: fix a compile warning with SMP=n

This patch fixes the following compile warning with CONFIG_SMP=n:

<-- snip -->

...
CC drivers/cpufreq/cpufreq.o
drivers/cpufreq/cpufreq.c: In function 'cpufreq_remove_dev':
drivers/cpufreq/cpufreq.c:696: warning: unused variable 'cpu_sys_dev'
...

<-- snip -->

@@ -693,8 +693,8 @@
 	unsigned int cpu = sys_dev->id;
 	unsigned long flags;
 	struct cpufreq_policy *data;
-	struct sys_device *cpu_sys_dev;
 #ifdef CONFIG_SMP
+	struct sys_device *cpu_sys_dev;
 	unsigned int j;
 #endif