[nlassert] Add GN support to build nlassert header

Change-Id: I772e2fd50f511f32052be5ee1337346215ddeef3

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000..17edf6f
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,19 @@
+# Copyright 2020 The Fuchsia Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+source_set("nlassert") {
+  public = [
+    "include/nlassert-config.h.in",
+    "include/nlassert.h",
+    "include/nlassert-internal.h",
+    "include/nlassert-nonproduction.h",
+    "include/nlassert-production.h",
+    "include/stdc/assert.h",
+  ]
+  public_configs = [ ":nlassert_config" ]
+}
+
+config("nlassert_config") {
+  include_dirs = [ "include" ]
+}