perf tools: Fallback to srcdir/Documentation/tips.txt

Some people don't install perf, but just use compiled version in the
source.  Fallback to lookup the source directory for those poor guys. :)

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452334589-8782-4-git-send-email-namhyung@kernel.org
[ Make perf_tip() return NULL for ENOENT, making the fallback to really take place ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>