disable-shell.inc: add oksh (#3829)

"Portable OpenBSD ksh, based on the Public Domain Korn Shell (pdksh)."

Project page: https://github.com/ibara/oksh

    $ pacman -Q oksh
    oksh 6.8.1-1
    $ pacman -Qlq oksh | grep bin/
    /usr/bin/
    /usr/bin/oksh

etc/inc/disable-shell.inc

@@ -8,6 +8,7 @@ blacklist ${PATH}/dash
 blacklist ${PATH}/fish
 blacklist ${PATH}/ksh
 blacklist ${PATH}/mksh
+blacklist ${PATH}/oksh
 blacklist ${PATH}/sh
 blacklist ${PATH}/tclsh
 blacklist ${PATH}/tcsh