Add ability to drive splines backwards

Change-Id: Ib34366ed155625fba56c4f72b9adce0615c6f12d
diff --git a/frc971/control_loops/drivetrain/BUILD b/frc971/control_loops/drivetrain/BUILD
index 1480e4e..a1b96be 100644
--- a/frc971/control_loops/drivetrain/BUILD
+++ b/frc971/control_loops/drivetrain/BUILD
@@ -106,6 +106,7 @@
         ":spline",
         ":trajectory",
         "//aos:init",
+        "//aos/util:math",
     ],
 )