[WATCHDOG] ixp2000: Fix build failure caused by missing include

This patch fixes a build failure[1] caused by the missing include of
timer.h and thus fixes the build failure.

Notably the build failure existed since October 2009! [2]

References:
[1] http://kisskb.ellerman.id.au/kisskb/buildresult/1983339/
[2] http://kisskb.ellerman.id.au/kisskb/buildresult/1351737/

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>