genirq: provide old request_irq() for CONFIG_GENERIC_HARDIRQ=n

Impact: Undo compile breakage for archs with CONFIG_GENERIC_HARDIRQ=n

The threaded interrupt handler patches changed request_irq from extern
to inline. Architectures which do not use the generic irq code still
have request_irq() as a global function and therefor fail to compile.

Keep the extern declaration for CONFIG_GENERIC_HARDIRQ=n

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