Make ruby_reset_timezone internal

It is used only in hash.c, when `ENV['TZ']` is set.

Author: nobu

internal/time.h

@@ -28,7 +28,8 @@ struct timeval rb_time_timeval(VALUE);
 RUBY_SYMBOL_EXPORT_BEGIN
 /* time.c (export) */
 void ruby_reset_leap_second_info(void);
-void ruby_reset_timezone(const char *);
 RUBY_SYMBOL_EXPORT_END
 
+void ruby_reset_timezone(const char *);
+
 #endif /* INTERNAL_TIME_H */