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

kernel/irq/internals.h

@@ -437,7 +437,9 @@ static inline void irq_remove_debugfs_entry(struct irq_desc *desc)
 # ifdef CONFIG_IRQ_DOMAIN
 void irq_domain_debugfs_init(struct dentry *root);
 # else
-static inline void irq_domain_debugfs_init(struct dentry *root);
+static inline void irq_domain_debugfs_init(struct dentry *root)
+{
+}
 # endif
 #else /* CONFIG_GENERIC_IRQ_DEBUGFS */
 static inline void irq_add_debugfs_entry(unsigned int irq, struct irq_desc *d)