Fix backwards spline feedback

We weren't handling the logic for feedback on backwards splines
correctly.

TODO: Clean up some of the backwards logic to make it possible to
follow.

Change-Id: I0c18b8c245010151356935dfcbf8babdb98d0846
diff --git a/frc971/control_loops/drivetrain/BUILD b/frc971/control_loops/drivetrain/BUILD
index b08e96f..4e6f373 100644
--- a/frc971/control_loops/drivetrain/BUILD
+++ b/frc971/control_loops/drivetrain/BUILD
@@ -643,6 +643,7 @@
         ":drivetrain_config",
         ":spline_goal_fbs",
         ":trajectory_fbs",
+        "//aos/util:math",
         "//frc971/control_loops:c2d",
         "//frc971/control_loops:dlqr",
         "//frc971/control_loops:hybrid_state_feedback_loop",