tools lib traceevent: Shut up plugins make message

Getting rid of following build output:

  $ make O=/tmp/build/perf -C tools/perf/ install-bin
  ...
  make[3]: Nothing to be done for `plugins'.
  make[2]: Nothing to be done for `plugins'.
  ...

which triggers when traceevent library needs to be rebuilt, but we have
plugins built already.

Adding extra 'plugins' target with nop which is visible and triggers in
both Makefile parts (for detached output directory (O=...) the
traceevent Makefile spawns sub make for the build itself).

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1388595050-23005-2-git-send-email-jolsa@redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>