Added missing system libraries and headers for bazel.

Change-Id: I966d28dd5a6677acc73c608a89efa30f8358b0dd
diff --git a/aos/common/util/BUILD b/aos/common/util/BUILD
index dedec24..d94e6e4 100644
--- a/aos/common/util/BUILD
+++ b/aos/common/util/BUILD
@@ -102,6 +102,7 @@
     '//third_party/eigen',
     '//aos/common:time',
     '//aos/common/logging',
+    '//external:libm',
   ],
 )