fix CONFIG_MMCONFIG=n build warning

arch/x86/kernel/acpi/boot.c:100: warning: 'acpi_mcfg_64bit_base_addr' defined
but not used

http://bugzilla.kernel.org/show_bug.cgi?id=11743



Signed-off-by: Pavel Vasilyev <linuxoid@tochka.ru>
Signed-off-by: Len Brown <len.brown@intel.com>