Include the private watchdog_dev.h header to pickup the prototypes for the
watchdog_dev_register/unregister functions.

This quiets the following sparse warnings:

warning: symbol 'watchdog_dev_register' was not declared. Should it be static?
warning: symbol 'watchdog_dev_unregister' was not declared. Should it be static?
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>