perf tools: Pass build flags to traceevent build

So the extra user build flags are propagated to libtraceevent.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: "Herton R. Krzesinski" <herton@redhat.com>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Cc: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
Link: http://lkml.kernel.org/r/20181016150614.21260-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>