[WATCHDOG] fix watchdog/at91rm9200_wdt.c compilation

This patch fixes the following compile error:

<--  snip  -->

...
  CC      drivers/watchdog/at91rm9200_wdt.o
at91rm9200_wdt.c:188: error: 'at91_wdt_ioctl' undeclared here (not in a 
make[3]: *** [drivers/watchdog/at91rm9200_wdt.o] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>