Re-constify pointer to current.

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

Change-Id: I861a438b55a377496dc7794e25592fd5a02555c3
diff --git a/inc/hf/cpu.h b/inc/hf/cpu.h
index 865713f..3b90b13 100644
--- a/inc/hf/cpu.h
+++ b/inc/hf/cpu.h
@@ -126,4 +126,4 @@
 void vcpu_init(struct vcpu *vcpu, struct vm *vm);
 void vcpu_on(struct vcpu *vcpu);
 void vcpu_off(struct vcpu *vcpu);
-size_t vcpu_index(struct vcpu *vcpu);
+size_t vcpu_index(const struct vcpu *vcpu);