watchdog: Make watchdog core configurable as module

Under some circumstances it may be desirable to configure
the watchdog core as module. Enable it.

As part of this change, mark pretimeout governors as depending
on the watchdog core. This is necessary to prevent governors
from being built into the kernel if the watchdog core is built
as module.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>