watchdog: fix watchdog-test.c build warning

Fix compiler warning by making the function static:

Documentation/watchdog/src/watchdog-test.c:34:6: warning: no previous prototype for 'term'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>