other minior internal header tweaks

These headers need no rewrite.  Just add some minor tweaks, like
addition of #include lines.  Mainly cosmetic.

TIMET_MAX_PLUS_ONE was deleted because the macro was used from only
one place (directly write expression there).

internal/io.h

@@ -9,14 +9,14 @@
  *             modify this file, provided that  the conditions mentioned in the
  *             file COPYING are met.  Consult the file for details.
  */
+#include "ruby/ruby.h"          /* for VALUE */
+#include "ruby/io.h"            /* for rb_io_t */
 
 /* io.c */
 void ruby_set_inplace_mode(const char *);
 void rb_stdio_set_default_encoding(void);
 VALUE rb_io_flush_raw(VALUE, int);
-#ifdef RUBY_IO_H
 size_t rb_io_memsize(const rb_io_t *);
-#endif
 int rb_stderr_tty_p(void);
 void rb_io_fptr_finalize_internal(void *ptr);
 #define rb_io_fptr_finalize rb_io_fptr_finalize_internal