Added dlqr pole placement library.

This is a wrapper around slicot.  Some day, we'll replace it...  The
code involved is mildly terrifying

Change-Id: I6e09f7f84d9e85bc12d446e38b0014e3d4df6e6f
diff --git a/frc971/control_loops/BUILD b/frc971/control_loops/BUILD
index 01dc25e..360097d 100644
--- a/frc971/control_loops/BUILD
+++ b/frc971/control_loops/BUILD
@@ -226,3 +226,14 @@
     ],
 )
 
+cc_library(
+    name = "dlqr",
+    hdrs = [
+        "dlqr.h",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//third_party/eigen",
+        "@slycot_repo//:slicot",
+    ],
+)