x86: fix sleep.c build error

fix:

arch/x86/kernel/acpi/sleep.c: In function ‘acpi_save_state_mem':
arch/x86/kernel/acpi/sleep.c:75: error: ‘stack_start' undeclared (first use in this function)
arch/x86/kernel/acpi/sleep.c:75: error: (Each undeclared identifier is reported
only once
arch/x86/kernel/acpi/sleep.c:75: error: for each function it appears in.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>