gpiolib: make lineevent_irq_thread static

The lineevent_irq_thread is not exported, so make it static
to fix the following warning:

drivers/gpio/gpiolib.c:654:13: warning: symbol 'lineevent_irq_thread' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>