irqchip: irq-xtensa-mx: include header for missing prototype

Add <linux/irqchips/xtensa-mx.h> to provide the function prototype to
prevent a build warning:

drivers/irqchip/irq-xtensa-mx.c:166:12: warning: no previous prototype for 'xtensa_mx_init_legacy' [-Wmissing-prototypes]
  166 | int __init xtensa_mx_init_legacy(struct device_node *interrupt_parent)

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <maz@kernel.org>
Message-Id: <20230920052139.10570-10-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>