docs: watchdog: fix obsolete include file reference in pcwd

The file linux/pcwd.h is not more present in the kernel
sources. Its information is now moved into the file:

include/uapi/linux/watchdog.h

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Link: https://lore.kernel.org/r/20210223111324.309285-1-f.suligoi@asem.it


Signed-off-by: Jonathan Corbet <corbet@lwn.net>