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 <[e-mail]>
Acked-by: Marc Zyngier <[e-mail]>
Cc: Chris Zankel <[e-mail]>
Cc: Max Filippov <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Max Filippov <[e-mail]>

drivers/irqchip/irq-xtensa-mx.c

···
 #include <linux/irqdomain.h>
 #include <linux/irq.h>
 #include <linux/irqchip.h>
+#include <linux/irqchip/xtensa-mx.h>
 #include <linux/of.h>
 
 #include <asm/mxregs.h>