x86: io_apic: Fix CONFIG_X86_IO_APIC=n breakage

Stupid me forgot to change the function name for the
CONFIG_X86_IO_APIC=n case in commit 23f9b267 (x86: apic: Move
probe_nr_irqs_gsi() into ioapic_init_mappings())

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>