perf tools: Do not display extra info when there is nothing to build

Even with fully built tree, we still display extra output when make is
invoked, like:

  $ make
    BUILD:   Doing 'make -j8' parallel build
    DESCEND  plugins
  make[3]: Nothing to be done for 'plugins/libtraceevent-dynamic-list'.

Changing the make descend directly to plugins directory, which quiets
those messages down.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Khuong <pvk@pvk.ca>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200507095024.2789147-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>