perf python: Add missing python/perf.so dependency for libperf

The python/perf.so compilation needs libperf ready, otherwise it fails:

  $ make python/perf.so JOBS=1
    BUILD:   Doing 'make -j1' parallel build
    GEN      python/perf.so
  gcc: error: /home/jolsa/kernel/linux-perf/tools/perf/lib/libperf.a: No such file or directory

Fixing this with by adding libperf dependency.

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: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20190901124822.10132-2-jolsa@kernel.org


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