Add missing reference for newer clang

We were triggering a copy when we shouldn't have been.

Change-Id: I280e788932a00c8feab92dadc272337b0af60520
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/y2019/vision/target_finder.cc b/y2019/vision/target_finder.cc
index 83d4a01..b16fba1 100644
--- a/y2019/vision/target_finder.cc
+++ b/y2019/vision/target_finder.cc
@@ -454,7 +454,7 @@
     // direction vector.
     ::Eigen::Vector2f smallest_point = polygon.contour[0];
     float smallest_value = outer_edge_vector.transpose() * smallest_point;
-    for (const ::Eigen::Vector2f point : polygon.contour) {
+    for (const ::Eigen::Vector2f &point : polygon.contour) {
       const float current_value = outer_edge_vector.transpose() * point;
       if (current_value < smallest_value) {
         smallest_value = current_value;