perf help: Fix a bug during strstart() conversion

perf help: Fix a bug during strstart() conversion

The commit a07cf3db29a0 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: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Taeung 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: a07cf3db29a0 ("tools include: Adopt strstarts() from the kernel")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>