perfetto: ensure that ftrace descriptor is only built in standalone

Change-Id: Ie96457deb83287186e4c9dedf00140596bdec762
diff --git a/protos/perfetto/trace/ftrace/BUILD.gn b/protos/perfetto/trace/ftrace/BUILD.gn
index bccb787..d6b4170 100644
--- a/protos/perfetto/trace/ftrace/BUILD.gn
+++ b/protos/perfetto/trace/ftrace/BUILD.gn
@@ -31,10 +31,12 @@
   generator_plugin_options = "wrapper_namespace=pbzero"
 }
 
-proto_descriptor("descriptor") {
-  generate_descriptor =
-      "$perfetto_root_path/protos/perfetto/trace/ftrace.descriptor"
-  sources = ftrace_proto_names
-  proto_in_dir = "$perfetto_root_path/protos"
-  proto_out_dir = "$perfetto_root_path/protos"
+if (perfetto_build_standalone) {
+  proto_descriptor("descriptor") {
+    generate_descriptor =
+        "$perfetto_root_path/protos/perfetto/trace/ftrace.descriptor"
+    sources = ftrace_proto_names
+    proto_in_dir = "$perfetto_root_path/protos"
+    proto_out_dir = "$perfetto_root_path/protos"
+  }
 }