ioapic: unexport ioapic_set_irq

There's no need to use ioapic_set_irq() outside of ioapic.c, so
make it static.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/ioapic.c b/hw/ioapic.c
index 335da6e..e3f8a46 100644
--- a/hw/ioapic.c
+++ b/hw/ioapic.c
@@ -94,7 +94,7 @@
     }
 }
 
-void ioapic_set_irq(void *opaque, int vector, int level)
+static void ioapic_set_irq(void *opaque, int vector, int level)
 {
     IOAPICState *s = opaque;