lib, traceevent: add PRINT_HEX_STR variant

Add support for the __print_hex_str() macro that was added for
tracing, so that user space tools such as perf can understand
it as well.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>