Re-constify pointer to current.

It doesn't actually get modified so can remain const.

Change-Id: I861a438b55a377496dc7794e25592fd5a02555c3
diff --git a/src/cpu.c b/src/cpu.c
index f87d78d..03e0074 100644
--- a/src/cpu.c
+++ b/src/cpu.c
@@ -156,7 +156,7 @@
 	sl_unlock(&vcpu->lock);
 }
 
-size_t vcpu_index(struct vcpu *vcpu)
+size_t vcpu_index(const struct vcpu *vcpu)
 {
 	return vcpu - vcpu->vm->vcpus;
 }