Fix insecure string formatting

core_unwind_elfutils.c: In function 'sr_parse_coredump':
core_unwind_elfutils.c:169:13: error: format not a string literal and no format arguments [-Werror=format-security]
             set_error(thread_arg.error_msg);
             ^

Signed-off-by: Martin Milata <mmilata@redhat.com>