genirq: Remove unneeded forward declaration

The protoype of irq_flow_handler_t is independent of irq_data, so remove
unneeded forward declaration.

Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240122085716.2999875-4-dawei.li@shingroup.cn