time: fix sys_timer_settime prototype

A small typo has crept into the y2038 conversion of the timer_settime
system call. So far this was completely harmless, but once we start
using the new version, this has to be fixed.

Fixes: 6ff84735 ("time: Change types to new y2038 safe __kernel_itimerspec")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>