perf config: fix bug in iskeychar()

[paulkim90]

perf config: fix bug in parsing 'man..*' config

To add new man viewer, configs like 'man.<tool>.cmd',
'man.<tool>.path' can be set into config file (~/.perfconfig).
But parsing config file is stopped because the config variable
contains '.' character i.e.

If setting 'man.xman.cmd' into config file,

    [man]
        gman.cmd = gman

when launching perf an error message is printed like below.

    Fatal: bad config file line 11 in /home/taeung/.perfconfig

So modify iskeychar() function to decide '.' character
as key character parsing config file.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Paul Kim <wogns3250@gmail.com>