add missing boundary

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/TruffleCextBuiltins.java

@@ -1392,6 +1392,7 @@ abstract static class MethodDescriptorRoot extends RootNode {
         @Child protected ReadIndexedArgumentNode readSelfNode;
         protected final PythonObjectFactory factory;
 
+        @TruffleBoundary
         protected MethodDescriptorRoot(PythonLanguage language, PythonObjectFactory factory, CallTarget callTarget) {
             super(language);
             this.factory = factory;