Use |is_fuchsia| in BUILD.gn

Change-Id: I8a977f8dcb210e999677754efa342d231f22a37b
diff --git a/include/BUILD.gn b/include/BUILD.gn
index 1082a78..d05d425 100644
--- a/include/BUILD.gn
+++ b/include/BUILD.gn
@@ -26,7 +26,7 @@
 }
 
 config("vulkan_public_config") {
-  if (current_os == "fuchsia") {
+  if (is_fuchsia) {
     defines = [ "VK_USE_PLATFORM_FUCHSIA=1" ]
   }
 }