watchdog: Enforce that at least one pretimeout governor is enabled

Since commit "watchdog: Use depends instead of select for pretimeout
governors", it was possible to enable pretimeout governors but keep all
of them disabled. Doing this results in the following build failure.

../drivers/watchdog/watchdog_pretimeout.c:
	In function ‘watchdog_register_governor’:
../drivers/watchdog/watchdog_pretimeout.c:139:26: error:
	‘WATCHDOG_PRETIMEOUT_DEFAULT_GOV’ undeclared
   if (!strncmp(gov->name, WATCHDOG_PRETIMEOUT_DEFAULT_GOV,

Since it does not make sense to enable pretimeout support but disable
all pretimeout governors, enforce that at least one of them is always
enabled.

Fixes: f627ac0e ("watchdog: Use depends instead of select for pretimeout governors")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>