x86/xen: Use correct #ifdef guard for xen_initdom_restore_msi()

x86/xen: Use correct #ifdef guard for xen_initdom_restore_msi()

The #ifdef check around the definition doesn't match the one around the
declaration, leading to a link failure when CONFIG_XEN_DOM0 is enabled
but CONFIG_XEN_PV_DOM0 is not:

x86_64-linux-ld: arch/x86/kernel/apic/msi.o: in function `arch_restore_msi_irqs':
msi.c:(.text+0x29a): undefined reference to `xen_initdom_restore_msi'

Change the declaration to use the same check that was already present
around the function definition.

Fixes: ae72f3156729 ("PCI/MSI: Make arch_restore_msi_irqs() less horrible.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20211215140209.451379-1-arnd@kernel.org