perf tools: Compile perf with libperf-in.o instead of libperf.a

There's no need for perf build to use libperf.a,
we can use directly libperf-in.o.

The libperf.a stays as a target if needed:

  $ make libperf.a
  ...
    CC       util/pmu.o
    CC       util/pmu-flex.o
    LD       util/libperf-in.o
    LD       libperf-in.o
    AR       libperf.a

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: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190213123246.4015-2-jolsa@kernel.org


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