Fix Ruby 4.1.0dev compatibility

Author: byroot

ext/json/ext/json.h

@@ -54,7 +54,7 @@ typedef unsigned char _Bool;
 #   define RUBY_TYPED_FROZEN_SHAREABLE 0
 #endif
 
-#ifndef HAVE_RUBY_TYPED_EMBEDDABLE
+#ifndef RUBY_TYPED_EMBEDDABLE
 # define RUBY_TYPED_EMBEDDABLE 0
 #endif