xen: mark xen_init_IRQ __init

xen_init_IRQ should be marked __init because it calls other functions
marked __init and is always called by functions marked __init (on both
x86 and arm).

Also remove the unused EXPORT_SYMBOL_GPL(xen_init_IRQ).

Both changes were introduced by "xen/arm: receive Xen events on ARM".
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/events.c

@@ -1802,7 +1802,7 @@ void xen_callback_vector(void)
 void xen_callback_vector(void) {}
 #endif
 
-void xen_init_IRQ(void)
+void __init xen_init_IRQ(void)
 {
 	int i;
 
@@ -1846,4 +1846,3 @@ void xen_init_IRQ(void)
 	}
 #endif
 }
-EXPORT_SYMBOL_GPL(xen_init_IRQ);