tools lib traceevent: Fix build failure on 32-bit arch

In my i386 build, it failed like this:

    CC       event-parse.o
  event-parse.c: In function 'print_str_arg':
  event-parse.c:3868:5: warning: format '%lu' expects argument of type 'long unsigned int',
                        but argument 3 has type 'uint64_t' [-Wformat]

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Javi Merino <javi.merino@arm.com>
Link: http://lkml.kernel.org/r/20150424020218.GF1905@sejong
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>