Add ibus-gtk[2|3].i686 to multilib_whitelist
    
    Some x86_64 users wish to install ibus-gtk3.i686 and ibus-gtk2.i686
    in the multilib systems but ibus.i686 is not required now.
    The users could install them with `dnf install ibus-gtk3.i686` when
    those modules are added to multilib_whitelist.
    
    https://pagure.io/releng/issue/7937
    
    Signed-off-by: Takao Fujiwara <tfujiwar@redhat.com>