openrisc: add missing header inclusion

Prevents build issue with updated toolchain

Reported-by: Jack Thomasson <jkt@moonlitsw.com>
Tested-by: Christian Svensson <blue@cmd.nu>
Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: Jonas Bonn <jonas@southpole.se>