MAINTAINERS: Add Guenter Roeck as co-maintainer of watchdog subsystem

Having a co-maintainer will enable us to be more flexible with
pull requests.

Cc: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

MAINTAINERS

@@ -14722,7 +14722,7 @@ F:	drivers/input/tablet/wacom_serial4.c
 
 WATCHDOG DEVICE DRIVERS
 M:	Wim Van Sebroeck <wim@iguana.be>
-R:	Guenter Roeck <linux@roeck-us.net>
+M:	Guenter Roeck <linux@roeck-us.net>
 L:	linux-watchdog@vger.kernel.org
 W:	http://www.linux-watchdog.org/
 T:	git git://www.linux-watchdog.org/linux-watchdog.git