tracing/hwlat: Make some internal symbols static

The sparse tool complains as follows:

kernel/trace/trace_hwlat.c:82:27: warning: symbol 'hwlat_single_cpu_data' was not declared. Should it be static?
kernel/trace/trace_hwlat.c:83:1: warning: symbol '__pcpu_scope_hwlat_per_cpu_data' was not declared. Should it be static?

This symbol is not used outside of trace_hwlat.c, so this commit
marks it static.

Link: https://lkml.kernel.org/r/20211021035225.1050685-1-bobo.shaobowang@huawei.com

Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>