Added missing system libraries and headers for bazel.

Change-Id: I966d28dd5a6677acc73c608a89efa30f8358b0dd
diff --git a/frc971/control_loops/BUILD b/frc971/control_loops/BUILD
index 75fda32..c220b96 100644
--- a/frc971/control_loops/BUILD
+++ b/frc971/control_loops/BUILD
@@ -89,6 +89,7 @@
   deps = [
     '//third_party/eigen',
     '//aos/common/controls:polytope',
+    '//external:libm',
   ],
 )