tracing: Fix __print_hex_dump scope

undef is needed for parsing __print_hex_dump in traceevent lib.

Link: http://lkml.kernel.org/r/1574762791-14883-1-git-send-email-piotrx.maziarz@linux.intel.com

Signed-off-by: Piotr Maziarz <piotrx.maziarz@linux.intel.com>
Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>