genirq/debugfs: Fix build for !CONFIG_IRQ_DOMAIN

Fix this build error:

kernel/irq/internals.h:440:20: error: inlining failed in call to always_inline
  'irq_domain_debugfs_init': function body not available
kernel/irq/debugfs.c:202:2: note: called from here
  irq_domain_debugfs_init(root_dir);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/alpine.LFD.2.20.1707041124000.1712@schleppi