Added missing system libraries and headers for bazel.

Change-Id: I966d28dd5a6677acc73c608a89efa30f8358b0dd
diff --git a/y2015/util/BUILD b/y2015/util/BUILD
index bf1d3ed..27c144d 100644
--- a/y2015/util/BUILD
+++ b/y2015/util/BUILD
@@ -5,6 +5,7 @@
   deps = [
     '//third_party/eigen',
     '//y2015:constants',
+    '//external:libm',
   ],
 )