perf help: Fix a bug during strstart() conversion

perf help: Fix a bug during strstart() conversion

The commit 8e99b6d4533c 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 <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Tested-by: Taeung Song <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Sihyeon Jang <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/20171114001542.GA16464@sejong
Fixes: 8e99b6d4533c ("tools include: Adopt strstarts() from the kernel")
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>