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 <ricardo.ribalda@gmail.com>
Reviewed-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Jan Beulich <JBeulich@suse.com>
Link: http://lkml.kernel.org/r/1422905231-16067-1-git-send-email-ricardo.ribalda@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/apic.h

@@ -213,7 +213,15 @@ void register_lapic_address(unsigned long address);
 extern void setup_boot_APIC_clock(void);
 extern void setup_secondary_APIC_clock(void);
 extern int APIC_init_uniprocessor(void);
+
+#ifdef CONFIG_X86_64
+static inline int apic_force_enable(unsigned long addr)
+{
+	return -1;
+}
+#else
 extern int apic_force_enable(unsigned long addr);
+#endif
 
 extern int apic_bsp_setup(bool upmode);
 extern void apic_ap_setup(void);