cpufreq: fix a -Wimplicit-function-declaration

happens if build with -lcpupower and kernel 4.7+

```
cpufreq-monitor-libcpufreq.c: In function 'cpufreq_monitor_libcpufreq_run':
cpufreq-monitor-libcpufreq.c:117:7: warning: implicit declaration of function 'cpupower_is_cpu_online' [-Wimplicit-function-declaration]
   if (cpupower_is_cpu_online (cpu)) {
       ^~~~~~~~~~~~~~~~~~~~~~
```

cpufreq/src/cpufreq-monitor-libcpufreq.c

@@ -98,6 +98,10 @@ cpufreq_monitor_libcpufreq_new (guint cpu)
         return CPUFREQ_MONITOR (monitor);
 }
 
+#if LINUX_VERSION_CODE > KERNEL_VERSION(4, 7, 0)
+extern int cpupower_is_cpu_online (unsigned int cpu);
+#endif
+
 static gboolean
 cpufreq_monitor_libcpufreq_run (CPUFreqMonitor *monitor)
 {