[pseudo] Fix the missing-field-initializers warning from f1ac00c, NFC

Author: hokein

clang-tools-extra/pseudo/lib/grammar/GrammarBNF.cpp

@@ -232,7 +232,7 @@ class GrammarBuilder {
         continue;
       }
 
-      Out.Sequence.push_back({Chunk});
+      Out.Sequence.push_back({Chunk, /*Attributes=*/{}});
     }
     return true;
   }