[fidl] Extend transports for overnet internals

Add a 'Transport="OvernetStream"' attribute, to gate code generation of
internal overnet types for interfaces.

Test: Compiled it

Change-Id: Ib377f02e2a07ed2b84b5cb611fe7c23b276952ae
diff --git a/system/host/fidl/lib/flat_ast.cpp b/system/host/fidl/lib/flat_ast.cpp
index a6dcccb..b742060 100644
--- a/system/host/fidl/lib/flat_ast.cpp
+++ b/system/host/fidl/lib/flat_ast.cpp
@@ -521,6 +521,7 @@
     }, {
         "Channel",
         "SocketControl",
+        "OvernetStream",
     }));
 }