Fixed root_type leaking in from included files.

Change-Id: I9378e375901f8a883eaeb2182955d690c358ed4b
Tested: on Linux.

src/idl_parser.cpp

@@ -2216,6 +2216,9 @@ CheckedError Parser::DoParse(const char *source,
                        name.c_str()));
         // We generally do not want to output code for any included files:
         if (!opts.generate_all) MarkGenerated();
+        // Reset root type just in case the included file had one, and the
+        // parent doesn't.
+        root_struct_def_ = nullptr;
         // This is the easiest way to continue this file after an include:
         // instead of saving and restoring all the state, we simply start the
         // file anew. This will cause it to encounter the same include