veyron/lib/bluetooth: Fix build on Mac by adding a missing build directive.

Change-Id: Ie999e43ca093bb513c6f280328358262222fd02c
diff --git a/lib/bluetooth/fd.go b/lib/bluetooth/fd.go
index 54766f3..328c351 100644
--- a/lib/bluetooth/fd.go
+++ b/lib/bluetooth/fd.go
@@ -1,3 +1,5 @@
+// +build linux
+
 package bluetooth
 
 // #include <stddef.h>