tools lib traceevent: Define _GNU_SOURCE in Makefile

tools lib traceevent: Define _GNU_SOURCE in Makefile

For the reasons stated on:

  commit a11c9cd
  Author: David Daney <david.daney@cavium.com>

    perf tools: Fix broken build by defining _GNU_SOURCE in Makefile

Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: David Daney <david.daney@cavium.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-e2nofbmj4uf0ykgsytxvt9pu@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>