cpufreq / governor: Fix problem with cpufreq_ondemand or cpufreq_conservative

Since commit 2aacdfff entitled "cpufreq: Move common part from governors
to separate file", whenever the drivers that depend on this new file
(cpufreq_ondemand or cpufreq_conservative) are built as modules, a new
module named cpufreq_governor is created because the Makefile includes
cpufreq_governor.o twice. As drivers/cpufreq/cpufreq_governor.c contains no
MODULE directives, the resulting module has no license specified, which
results in logging of a "module license 'unspecified' taints kernel". In
addition, a number of globals are exported GPL only, and are therefore
not available. This fix establishes a new boolean configuration variable
that forces cpufreq_governor.o to be linked into the kernel whenever
either cpufreq_ondemand or cpufreq_conservative is selected.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>