openrisc: add local64.h to fix blk-iocost build

As of 5.10 OpenRISC allyesconfig builds fail with the following error.

    $ make ARCH=openrisc CROSS_COMPILE=or1k-elf- block/blk-iocost.o
      CALL    scripts/checksyscalls.sh
      CALL    scripts/atomic/check-atomics.sh
      CC      block/blk-iocost.o
    block/blk-iocost.c:183:10: fatal error: asm/local64.h: No such file or directory
      183 | #include <asm/local64.h>
	  |          ^~~~~~~~~~~~~~~
    compilation terminated.

The new include of local64.h was added in commit 5e124f74
("blk-iocost: use local[64]_t for percpu stat") by Tejun.

Adding the generic version of local64.h to OpenRISC fixes the build
issue.

Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>