soc: litex: Remove duplicated header file inclusion

The header file <linux/errno.h> is already included above and can be
removed here.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>