Fix comparison operator signature

New toolchain complains, and it's kind of a weird thing to have.

Change-Id: Id2ab7d0f588a4adcbcfc28735e4fd0013bfb8600
Signed-off-by: Brian Silverman <bsilver16834@gmail.com>
diff --git a/y2018/control_loops/superstructure/arm/graph.h b/y2018/control_loops/superstructure/arm/graph.h
index 5d49dd0..2300d6a 100644
--- a/y2018/control_loops/superstructure/arm/graph.h
+++ b/y2018/control_loops/superstructure/arm/graph.h
@@ -54,7 +54,7 @@
 
     T *get() { return value_; }
 
-    bool operator<(const QueueEntry &o) { return *value_ < *o.value_; }
+    bool operator<(const QueueEntry &o) const { return *value_ < *o.value_; }
 
    private:
     friend class IntrusivePriorityQueue;