fix const-ness in deepidexample

Signed-off-by: Peter Hillman <[email protected]>

src/examples/deepidexample.cpp

@@ -56,7 +56,7 @@ struct Rgbaz
     //
     // depth sort (use ID to resolve ties)
     //
-    bool operator<(const Rgbaz& other)
+    bool operator<(const Rgbaz& other) const
     {
         if (z < other.z) { return true; }
         if (z > other.z) { return false; }