m68k: Do not rely on magic indirect includes

commit: 8f945a33 (genirq: Move kstat_incr_irqs_this_cpu() to core)
unearthed the following:

arch/m68k/kernel/ints.c:34:15: error: variable 'auto_irq_chip' has initializer but incomplete type
arch/m68k/kernel/ints.c:35:2: error: unknown field 'name' specified in initializer
arch/m68k/kernel/ints.c:35:2: warning: excess elements in struct initializer [enabled by default]

The reason is that this file requires linux/irq.h and magically
pulled that in via linux/kernel_stat.h

The commit above got rid of the pointless include of linux/irq.h in
linux/kernel_stat.h and therefor broke the build.

Include linux/irq.h

Reported-by:  <fengguang.wu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>