os: add netbsd constraint value

os/BUILD

diff --git a/os/BUILD b/os/BUILD
index 53fec81..e35460a 100644
--- a/os/BUILD
+++ b/os/BUILD
@@ -19,6 +19,11 @@
 )
 
 constraint_value(
+    name = "netbsd",
+    constraint_setting = ":os",
+)
+
+constraint_value(
     name = "openbsd",
     constraint_setting = ":os",
 )