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>