irqdomain: Include internals.h for function prototypes

irq_domain_debugfs_init() is defined in irqdomain.c, but the
declaration is in a header that is not included here:

kernel/irq/irqdomain.c:1965:13: error: no previous prototype for 'irq_domain_debugfs_init' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230516200432.554240-1-arnd@kernel.org