drivers: interrupt_controller: gd32_exti: isr may be unused

The gd32_exti_isr function may be unused if the GPIO driver is not
enabled but EXTI is (no IRQ will connect to it). This may be improved in
the future by requiring explicit enablement of the exti DT node.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>