Add IEnumerator`1 to MonoDefaults struct

mono/metadata/class-internals.h

@@ -1113,6 +1113,7 @@ typedef struct {
 	MonoClass *customattribute_data_class;
 	MonoClass *critical_finalizer_object; /* MAYBE NULL */
 	MonoClass *generic_ireadonlylist_class;
+	MonoClass *generic_ienumerator_class;
 	MonoClass *threadpool_wait_callback_class;
 	MonoMethod *threadpool_perform_wait_callback_method;
 	MonoClass *console_class;

mono/metadata/domain.c

@@ -771,6 +771,8 @@ mono_init_internal (const char *filename, const char *exe_filename, const char *
 	        mono_defaults.corlib, "System.Collections.Generic", "IList`1");
 	mono_defaults.generic_ireadonlylist_class = mono_class_load_from_name (
 	        mono_defaults.corlib, "System.Collections.Generic", "IReadOnlyList`1");
+	mono_defaults.generic_ienumerator_class = mono_class_load_from_name (
+	        mono_defaults.corlib, "System.Collections.Generic", "IEnumerator`1");
 
 	mono_defaults.threadpool_wait_callback_class = mono_class_load_from_name (
 		mono_defaults.corlib, "System.Threading", "_ThreadPoolWaitCallback");