Remove runtime malloc's from SplineDrivetrain

This lets drivetrain_lib_test pass with die_on_malloc enabled.

Change-Id: If85b2ee5635fd82ba4aaf2ef18b4736cb6099d27
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/frc971/control_loops/drivetrain/distance_spline.h b/frc971/control_loops/drivetrain/distance_spline.h
index 91b9898..f445fa4 100644
--- a/frc971/control_loops/drivetrain/distance_spline.h
+++ b/frc971/control_loops/drivetrain/distance_spline.h
@@ -123,7 +123,7 @@
   // The spline we are converting to a distance.
   aos::SizedArray<Spline, kMaxSplines> splines_;
   // An interpolation table of distances evenly distributed in alpha.
-  const absl::Span<const float> distances_;
+  absl::Span<const float> distances_;
 };
 
 }  // namespace drivetrain