[CPUFREQ] reduce scope of ACPI_PSS_BIOS_BUG_MSG[]

This symbol doesn't need file-global scope.

Cc: "Zhang, Rui" <rui.zhang@intel.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Langsdorf, Mark <mark.langsdorf@amd.com>
Cc: Leo Milano <lmilano@gmx.net>
Cc: Thomas Renninger <trenn@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Jones <davej@redhat.com>