x86: io-apic - declare irq_cfg_lock for SPARSE_IRQ only

We use irq_cfg_lock lock in SPARSE_IRQ only context so
move it under #ifdef and compiler will be happy.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>