Fix merged perfetto trace

The change to group the ftrace protos broke the merged proto because
I had accidentally duplicated some entries in gen_merged_protos. This
fixes the issue and adds a build target so this can't happen in future.

Change-Id: Iba2a3dfa1836f2c36bdd65fea67b2518f1b03eeb
diff --git a/BUILD.gn b/BUILD.gn
index ca8a9c3..307f3aa 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -54,6 +54,7 @@
       ":traced",
       ":traced_probes",
       "protos/perfetto/config:merged_config",  # For syntax-checking the proto.
+      "protos/perfetto/trace:merged_trace",  # For syntax-checking the proto.
       "src/ipc/protoc_plugin:ipc_plugin($host_toolchain)",
       "tools:protoc_helper",
     ]