kernel/wdog: Compile in kern_wdog.c by default.

kernel/wdog: Compile in kern_wdog.c by default.

Actually it was always compiled in by default but the code depended
on the WATCHDOG_ENABLED option which is not in the GENERIC kernels.

Simply remove the WATCHDOG_ENABLE option. The code is small, it does
nothing by default than initializing a lock, a /dev/wdog and a
callout, and it also makes it easier for people who want to use
ichwd(4) by simply kldloading it.