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>

arch/x86/kernel/acpi/sleep.c

@@ -86,7 +86,9 @@ int acpi_save_state_mem(void)
 	saved_magic = 0x12345678;
 #else /* CONFIG_64BIT */
 	header->trampoline_segment = setup_trampoline() >> 4;
+#ifdef CONFIG_SMP
 	stack_start.sp = temp_stack + 4096;
+#endif
 	initial_code = (unsigned long)wakeup_long64;
 	saved_magic = 0x123456789abcdef0;
 #endif /* CONFIG_64BIT */