x86: mp build fix

fix:

 drivers/built-in.o: In function `acpi_pci_irq_enable':
 : undefined reference to `mp_config_acpi_gsi'
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/mpspec.h

@@ -49,8 +49,17 @@ extern void mp_override_legacy_irq(u8 bus_irq, u8 polarity, u8 trigger,
 extern void mp_config_acpi_legacy_irqs(void);
 extern int mp_register_gsi(u32 gsi, int edge_level, int active_high_low);
 extern void MP_intsrc_info(struct mpc_config_intsrc *m);
+#ifdef CONFIG_X86_IO_APIC
 extern int mp_config_acpi_gsi(unsigned char number, unsigned int devfn, u8 pin,
 				u32 gsi, int triggering, int polarity);
+#else
+static inline int
+mp_config_acpi_gsi(unsigned char number, unsigned int devfn, u8 pin,
+		   u32 gsi, int triggering, int polarity)
+{
+	return 0;
+}
+#endif
 #endif /* CONFIG_ACPI */
 
 #define PHYSID_ARRAY_SIZE	BITS_TO_LONGS(MAX_APICS)