x86/dumpstack: Make show_trace_log_lvl() static

show_trace_log_lvl() is not used by other compilation units so make it
static and remove the declaration from the header file.

Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201113133943.GA136221@rlk