Add missing Ruby version guards for rb_data_define()'s specs

Author: andrykonchin

optional/capi/ext/struct_spec.c

@@ -62,6 +62,7 @@ static VALUE struct_spec_rb_struct_size(VALUE self, VALUE st) {
   return rb_struct_size(st);
 }
 
+#if defined(RUBY_VERSION_IS_3_3)
 /* Only allow setting three attributes, should be sufficient for testing. */
 static VALUE struct_spec_rb_data_define(VALUE self, VALUE superclass,
   VALUE attr1, VALUE attr2, VALUE attr3) {
@@ -76,6 +77,7 @@ static VALUE struct_spec_rb_data_define(VALUE self, VALUE superclass,
 
   return rb_data_define(superclass, a1, a2, a3, NULL);
 }
+#endif
 
 void Init_struct_spec(void) {
   VALUE cls = rb_define_class("CApiStructSpecs", rb_cObject);
@@ -88,7 +90,9 @@ void Init_struct_spec(void) {
   rb_define_method(cls, "rb_struct_define_under", struct_spec_rb_struct_define_under, 5);
   rb_define_method(cls, "rb_struct_new", struct_spec_rb_struct_new, 4);
   rb_define_method(cls, "rb_struct_size", struct_spec_rb_struct_size, 1);
+#if defined(RUBY_VERSION_IS_3_3)
   rb_define_method(cls, "rb_data_define", struct_spec_rb_data_define, 4);
+#endif
 }
 
 #ifdef __cplusplus