x86, irq: Move irq_remapping_enabled declaration to iommu code

Remove the last left-over from this flag from x86 code.

Signed-off-by: Joerg Roedel <joro@8bytes.org>
Acked-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>