cpufreq: Make cpufreq_global_kobject static

The cpufreq_global_kobject is only used internally by cpufreq.c
after commit 2361be23

 ("cpufreq: Don't create empty
/sys/devices/system/cpu/cpufreq directory").

Make it static.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
[ rjw: Add empty line after cpufreq_global_kobject definition ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>