irqchip/st: Mark st_irq_syscfg_resume() __maybe_unused

When CONFIG_PM_SLEEP is disabled, the compiler raises a warning on
st_irq_syscfg_resume:

drivers/irqchip/irq-st.c:183:12: warning: 'st_irq_syscfg_resume' defined
but not used [-Wunused-function]
 static int st_irq_syscfg_resume(struct device *dev)
             ^~~~~~~~~~~~~~~~~~~~

Annotate the function with __maybe_unused.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/20161217002927.31947-1-jeremy.lefaure@lse.epita.fr


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>