Simplify proto descriptor rules

Remove a couple of unnecessary path mangling steps and
fix ftrace.descriptor generation to be in the correct place.

Bug: 139319295
Change-Id: I7165ee70f8051ef50c511e17c3dbf47986ec1f1a
diff --git a/BUILD.gn b/BUILD.gn
index b14cf0c..beee2f9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -81,6 +81,9 @@
         # The diff testing framework depends on these descriptors.
         "protos/perfetto/metrics:descriptor",
         "protos/perfetto/trace:descriptor",
+
+        # Used in the when updating the ftrace protos
+        "protos/perfetto/trace/ftrace:descriptor",
       ]
       if (is_linux || is_android) {
         deps += [ "tools/busy_threads" ]