hw/m68k/next-cube: Make next_irq() function static

The next_irq() function is global, but isn't actually used anywhere
outside next-cube.c. Make it static.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210115201206.17347-2-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>

hw/m68k/next-cube.c

@@ -724,7 +724,7 @@ static const MemoryRegionOps dma_ops = {
  * TODO: set the shift numbers as values in the enum, so the first switch
  * will not be needed
  */
-void next_irq(void *opaque, int number, int level)
+static void next_irq(void *opaque, int number, int level)
 {
     M68kCPU *cpu = opaque;
     int shift = 0;

include/hw/m68k/next-cube.h

@@ -42,6 +42,4 @@ enum next_irqs {
     NEXT_SND_I
 };
 
-void next_irq(void *opaque, int number, int level);
-
 #endif /* NEXT_CUBE_H */