watchdog.h: Fix compile time error on disabling watchdog handling

There's a compile time error that we didn't catch since the
board defaults as used by the build bot won't expose it.

Just make watchdog_off() a no-op statement so there aren't any
stray semicolons in the preprocessor output.

Change-Id: Ib5595e7e8aa91ca54bc8ca30a39b72875c961464
Reported-by: 'lautriv' on irc.freenode.net/#coreboot
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/2627
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/include/watchdog.h

@@ -4,7 +4,7 @@
 #if CONFIG_USE_WATCHDOG_ON_BOOT
 void watchdog_off(void);
 #else
-#define watchdog_off()
+#define watchdog_off() while(0) {}
 #endif
 
 #endif /* WATCHDOG_H */