Security: exclude default-fonts-core-math
    
    See a90d590e00f1c972235dbb5793aa3eb81840c85e - this is the same
    fix, for the security image. As with LXDE, since
    https://fedoraproject.org/wiki/Changes/ImproveDefaultFontHandling
    we need to also drop default-fonts-core-math if we want to drop
    stix-fonts, or the image build fails.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>