x86/apic: Fix the devicetree build in certain configs

x86/apic: Fix the devicetree build in certain configs

Without this patch:

  LD      init/built-in.o
  arch/x86/built-in.o: In function `dtb_lapic_setup': kernel/devicetree.c:155:
  undefined reference to `apic_force_enable'
  Makefile:923: recipe for target 'vmlinux' failed
  make: *** [vmlinux] Error 1

Signed-off-by: Ricardo Ribalda Delgado <[email protected]>
Reviewed-by: Maciej W. Rozycki <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Jan Beulich <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>