This symbol is not used outside of trace.c, so marks it static.

Fix the following sparse warning:

 drivers/infiniband/hw/hfi1/trace.c:491:23: warning: symbol 'hist' was not declared. Should it be static?

Link: https://lore.kernel.org/r/1630921723-21545-1-git-send-email-jiapeng.chong@linux.alibaba.comReported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: chongjiapeng <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>