Fix compilation error with C23

This fallback already defined at ruby/internal/stdbool.h

Author: hsbt

ext/json/ext/generator/generator.h

@@ -6,12 +6,10 @@
 
 #include "ruby.h"
 
-#ifdef HAVE_STDBOOL_H
-#include <stdbool.h>
-#else
-/* This is the fallback definition from Ruby 3.0.5. */
+#ifndef RBIMPL_STDBOOL_H
+# /* This is the fallback definition from Ruby 3.0.5. */
 typedef unsigned char _Bool
-#define bool _Bool
+# define bool _Bool
 #endif
 
 #ifdef HAVE_RUBY_RE_H