perf tools: Include tools/lib/api/ in MANIFEST

When 553873e1 renamed tools/lib/lk to tools/lib/api we forgot to
do the switch in tools/perf/MANIFEST, breaking tarball building:

  [acme@ssdandy linux]$ make perf-targz-src-pkg
    TAR
  [acme@ssdandy linux]$ tar xf perf-3.13.0-rc4.tar.gz -C /tmp/tmp.OgdYyvp77p/
  [acme@ssdandy linux]$ make -C /tmp/tmp.OgdYyvp77p/perf-3.13.0-rc4/tools/perf
  make: Entering directory
  `/tmp/tmp.OgdYyvp77p/perf-3.13.0-rc4/tools/perf'
    BUILD:   Doing 'make -j8' parallel build
    FLEX     util/pmu-flex.c
    CC       util/evlist.o
    CC       util/evsel.o
  util/evsel.c:12:28: fatal error: api/fs/debugfs.h: No such file or directory compilation terminated.
  In file included from util/cache.h:5:0,
  <SNIP>

Fix it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-1wwjs01rt3xbyhn6kjl2gfs9@git.kernel.org


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