cpufreq / e_powersaver: Fix linker error when ACPI processor is a module

on i386:
CONFIG_ACPI_PROCESSOR=m
CONFIG_X86_E_POWERSAVER=y

drivers/built-in.o: In function `eps_cpu_init.part.8':
e_powersaver.c:(.text.unlikely+0x2243): undefined reference to `acpi_processor_register_performance'
e_powersaver.c:(.text.unlikely+0x22a2): undefined reference to `acpi_processor_unregister_performance'
e_powersaver.c:(.text.unlikely+0x246b): undefined reference to `acpi_processor_get_bios_limit'

X86_E_POWERSAVER should also depend on ACPI_PROCESSOR.

Signed-off-by: Rafal Bilski <rafalbilski@interia.pl>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>