x86: export vector_used_by_percpu_irq

Impact: build fix

lguest can be built as a module and makes use of this new symbol:

ERROR: "vector_used_by_percpu_irq" [drivers/lguest/lg.ko] undefined!

export it.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/irq.c

@@ -9,6 +9,7 @@
 #include <asm/apic.h>
 #include <asm/io_apic.h>
 #include <asm/smp.h>
+#include <asm/irq.h>
 
 atomic_t irq_err_count;
 
@@ -190,3 +191,5 @@ u64 arch_irq_stat(void)
 #endif
 	return sum;
 }
+
+EXPORT_SYMBOL_GPL(vector_used_by_percpu_irq);