doxygen: Put Print Support in IO

Update #3706.

cpukit/include/rtems/print.h

@@ -31,6 +31,8 @@ typedef struct rtems_printer rtems_printer;
 /**
  * @defgroup RTEMSPrintSupport RTEMS Print Support
  *
+ * @ingroup LibIO
+ *
  * This module contains all methods and support related to providing the user
  * with an interface to the kernel level print support.
  */