genirq/msi: Make __msi_domain_alloc_irqs() static

Nothing outside of the core code requires this.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20221111122014.004725919@linutronix.de