debug_counter.h must be self-contained

Include what is necessary.

debug_counter.h

@@ -341,6 +341,10 @@ RB_DEBUG_COUNTER(load_path_is_not_realpath)
 #ifndef RUBY_DEBUG_COUNTER_H
 #define RUBY_DEBUG_COUNTER_H 1
 
+#include "ruby/config.h"
+#include <stddef.h>             /* for size_t */
+#include "ruby/ruby.h"          /* for VALUE */
+
 #if !defined(__GNUC__) && USE_DEBUG_COUNTER
 #error "USE_DEBUG_COUNTER is not supported by other than __GNUC__"
 #endif