Small cleanups preparing for fet12v2 code

Change-Id: I3c0da058c4aef78bd7cc7590824f54ca664bfbae
diff --git a/third_party/GSL/BUILD b/third_party/GSL/BUILD
index 10fe510..62a28f3 100644
--- a/third_party/GSL/BUILD
+++ b/third_party/GSL/BUILD
@@ -1,8 +1,11 @@
 licenses(["notice"])
 
+load("//tools:environments.bzl", "mcu_cpus")
+
 cc_library(
     name = "GSL",
     hdrs = glob(["include/**"]),
+    compatible_with = mcu_cpus,
     includes = ["include"],
     visibility = ["//visibility:public"],
 )