irqchipr/i8259: Mark i8259_of_init() static

i8259_of_init() is only used as an initcall and does not need to be global,
so mark it static to avoid:

drivers/irqchip/irq-i8259.c:343:12: warning: no previous prototype for 'i8259_of_init' [-Wmissing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230810123404.1222332-3-arnd@kernel.org