Used external roborio compiler instead of local compiler.

Change-Id: Ifec0d68b0b51a822c906bc4f52320004ee2f5d84
diff --git a/aos/common/util/BUILD b/aos/common/util/BUILD
index d94e6e4..e2c6b8c 100644
--- a/aos/common/util/BUILD
+++ b/aos/common/util/BUILD
@@ -102,7 +102,7 @@
     '//third_party/eigen',
     '//aos/common:time',
     '//aos/common/logging',
-    '//external:libm',
+    '//debian:libm',
   ],
 )