gpio: lpc18xx: fix GPIO controller driver build as a module

The problem is reported for allmodconfig build setup:

ERROR: "irq_chip_retrigger_hierarchy" [drivers/gpio/gpio-lpc18xx.ko] undefined!
make[2]: *** [scripts/Makefile.modpost:92: __modpost] Error 1
make[1]: *** [Makefile:1271: modules] Error 2

My testing in runtime shows that it is sufficient to remove .irq_retrigger
callback, which is assigned to unexported irq_chip_retrigger_hierarchy()
function, I did't observe any regressions, and thus apparently it is a
better fix rather than exporting the function defined in kernel/irq/chip.c
(see commit 52b2a05fa7c8 ("genirq: Export IRQ functions for module use"))
or sticking the GPIO controller driver build to built-in option only.

Reported-by: kbuild test robot <[e-mail]>
Fixes: 5ddabfe8d3de ("gpio: lpc18xx: add GPIO pin interrupt controller support")
Signed-off-by: Vladimir Zapolskiy <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpio-lpc18xx.c

···
 #include <linux/gpio/driver.h>
 #include <linux/io.h>
 #include <linux/irqdomain.h>
-#include <linux/irqchip.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
···
 	.irq_unmask	= lpc18xx_gpio_pin_ic_unmask,
 	.irq_eoi	= lpc18xx_gpio_pin_ic_eoi,
 	.irq_set_type	= lpc18xx_gpio_pin_ic_set_type,
-	.irq_retrigger	= irq_chip_retrigger_hierarchy,
 	.flags		= IRQCHIP_SET_TYPE_MASKED,
 };