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

drivers/gpio/gpiolib.c

@@ -653,7 +653,7 @@ static const struct file_operations lineevent_fileops = {
 #endif
 };
 
-irqreturn_t lineevent_irq_thread(int irq, void *p)
+static irqreturn_t lineevent_irq_thread(int irq, void *p)
 {
 	struct lineevent_state *le = p;
 	struct gpioevent_data ge;