[cobalt] Don't rely on fuchsia gni files

Change-Id: Ia46f8e9693c33be545f02ca4e5f1c304bb25fdef
diff --git a/BUILD.gn b/BUILD.gn
index cca813d..9ac4974 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,7 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/fuchsia/sdk.gni")
+if (is_fuchsia) {
+  import("//build/fuchsia/sdk.gni")
+}
 
 config("gtest_private_config") {
   visibility = [ ":*" ]