The commit 8e99b6d4 changed prefixcmp() to strstart() but missed to
change the return value in some place.  It makes perf help print
annoying output even for sane config items like below:

  $ perf help
  '.root': unsupported man viewer sub key.
  ...
Reported-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NNamhyung Kim <namhyung@kernel.org>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: NTaeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Sihyeon Jang <uneedsihyeon@gmail.com>
Cc: kernel-team@lge.com
Link: http://lkml.kernel.org/r/20171114001542.GA16464@sejong
Fixes: 8e99b6d4 ("tools include: Adopt strstarts() from the kernel")
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>