perf tools: Fix output directory of Documentation/

The OUTPUT directory is wrongly determind leading to:

 make[3]: *** No rule to make target `.../.build/perf/PERF-VERSION-FILE'.  Stop.

Fixing this by using the generic approach in script/Makefile.include.

Signed-off-by: Robert Richter <robert.richter@calxeda.com>
Link: http://lkml.kernel.org/r/1367865614-30876-1-git-send-email-rric@kernel.org


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