irqchip/irq-pic32-evic: Add missing 'static' to internal function

Fix build error reported by gcc 12:

  drivers/irqchip/irq-pic32-evic.c:164:5: error: no previous prototype for ‘pic32_irq_domain_xlate’ [-Werror=missing-prototypes]
    164 | int pic32_irq_domain_xlate(struct irq_domain *d, struct device_node *ctrlr,
        |     ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240717-irq-pic32-evic-fix-build-static-v1-1-5129085589c6@bootlin.com