x86/hvm: fix vIO-APIC build without IRQ0_SPECIAL_ROUTING

x86/hvm: fix vIO-APIC build without IRQ0_SPECIAL_ROUTING

pit_channel0_enabled needs to be guarded with IRQ0_SPECIAL_ROUTING
since it's only used when the special handling of ISA IRQ 0 is
enabled. However such helper being a single line it's better to just
inline it directly in vioapic_deliver where it's used.

No functional change.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>