Split up zeroing library BUILD rules

There was no good reason for one build rule to have both the interface
and a bunch of implementations; split it up and make the relevant users
depend on the libraries directly.

Change-Id: If6a0aa49dd7ea588b7325604658bc0594db4aac0
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/y2018/control_loops/superstructure/arm/arm.h b/y2018/control_loops/superstructure/arm/arm.h
index 0b0a6a4..a9cf614 100644
--- a/y2018/control_loops/superstructure/arm/arm.h
+++ b/y2018/control_loops/superstructure/arm/arm.h
@@ -6,6 +6,7 @@
 #include "frc971/control_loops/double_jointed_arm/ekf.h"
 #include "frc971/control_loops/double_jointed_arm/graph.h"
 #include "frc971/control_loops/double_jointed_arm/trajectory.h"
+#include "frc971/zeroing/pot_and_absolute_encoder.h"
 #include "frc971/zeroing/zeroing.h"
 #include "y2018/constants.h"
 #include "y2018/control_loops/superstructure/arm/generated_graph.h"