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 52b2a05f

 ("genirq: Export IRQ functions for module use"))
or sticking the GPIO controller driver build to built-in option only.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 5ddabfe8

 ("gpio: lpc18xx: add GPIO pin interrupt controller support")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>