rtos: fixes warning

Change-Id: I45db15b16b52c71009d8830985f42ac88eabe160
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/1209
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>