irqchip/irq-msi-lib: Fix build with PCI disabled

The armada-370-xp irqchip fails in some randconfig builds because
of a missing declaration:

In file included from drivers/irqchip/irq-armada-370-xp.c:23:
include/linux/irqchip/irq-msi-lib.h:25:39: error: 'struct msi_domain_info' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]

Add a forward declaration for the msi_domain_info structure.

[ tglx: Fixed up the subsystem prefix. Is it really that hard to get right? ]

Fixes: e51b2743 ("irqchip: Make irq-msi-lib.h globally available")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/all/20250710080021.2303640-1-arnd@kernel.org