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: ae72f315 ("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

arch/x86/include/asm/xen/hypervisor.h

@@ -59,7 +59,7 @@ static inline bool __init xen_x2apic_para_available(void)
 
 struct pci_dev;
 
-#ifdef CONFIG_XEN_DOM0
+#ifdef CONFIG_XEN_PV_DOM0
 bool xen_initdom_restore_msi(struct pci_dev *dev);
 #else
 static inline bool xen_initdom_restore_msi(struct pci_dev *dev) { return true; }