watchdog: gef_wdt: include fs.h

Add missing include "linux/fs.h".
This fixes compile failure.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Martyn Welch <martyn.welch@ge.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>