rtos: declare local symbols as static

rtos: declare local symbols as static

Functions and variables that are not used outside the file should
be declared as static.

Change-Id: I9731a35496cd1c7421563c8961da5fa0e3cc71c3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5894
Tested-by: jenkins
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>