tools lib traceevent: Add install_headers target

tools lib traceevent: Add install_headers target

Adding install_headers target to install all headers
under 'include/traceevent' path, like:

  $ make DESTDIR=/tmp/krava prefix=/usr install_headers
  $ find /tmp/krava/ -type f
  /tmp/krava/usr/include/traceevent/kbuffer.h
  /tmp/krava/usr/include/traceevent/event-utils.h
  /tmp/krava/usr/include/traceevent/event-parse.h

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-if70lj3zhdc3csdqm5webjvc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>