irqchip/st: Mark st_irq_syscfg_resume() __maybe_unused

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 <[email protected]>
Cc: Marc Zyngier <[email protected]>
Cc: Jason Cooper <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>