Add another overload for MergeFlatbuffers

Change-Id: Ib167e0f72e61134a6e65d0923305848ca2e5d8c4
diff --git a/aos/flatbuffer_merge.h b/aos/flatbuffer_merge.h
index 5535763..ccbdd30 100644
--- a/aos/flatbuffer_merge.h
+++ b/aos/flatbuffer_merge.h
@@ -63,6 +63,14 @@
 }
 
 template <class T>
+inline flatbuffers::Offset<T> MergeFlatBuffers(
+    const T *fb1, const T *fb2, flatbuffers::FlatBufferBuilder *fbb) {
+  return MergeFlatBuffers<T>(reinterpret_cast<const flatbuffers::Table *>(fb1),
+                             reinterpret_cast<const flatbuffers::Table *>(fb2),
+                             fbb);
+}
+
+template <class T>
 inline flatbuffers::Offset<T> CopyFlatBuffer(
     const T *t1, flatbuffers::FlatBufferBuilder *fbb) {
   return MergeFlatBuffers<T>(reinterpret_cast<const flatbuffers::Table *>(t1),