Current LITEX_LAST_IRQ looks like a typo that blocks compilation of
`arty_a7/knsh` configuration.
This fixes the build but I have no such device for test.
Found it was LITEX_IRQ_LAST before commit #ee84ea3 so likely typo was
introduced by then.
Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>