x86/apic: Always provide irq_compose_msi_msg() method for vector domain

This shouldn't be dependent on PCI_MSI. HPET and I/O-APIC can deliver
interrupts through MSI without having any PCI in the system at all.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-10-dwmw2@infradead.org