Add grub2-mount to grub2-tools-minimal subpackage
    
    os-prober 1.75 dropped all the code for handling device mapper
    directly in favor of only supporting the use of grub2-mount.
    
    Thus, we now need grub2-mount to be built and packaged so that
    os-prober can depend on it. We ship it in the grub2-tools-minimal
    package to avoid creating a dependency loop between grub2-tools and
    os-prober.
    
    Resolves: rhbz#1471267
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com>