irqchip/mips-gic: Mark gic_irq_domain_free() static

This function is only used locally and should be static to avoid a warning:

drivers/irqchip/irq-mips-gic.c:560:6: error: no previous prototype for 'gic_irq_domain_free' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230810123404.1222332-2-arnd@kernel.org