spec: Only call ldconfig on RHEL7
    
    Since Fedora 28 (our minimum supported build), ldconfig is called
    automatically for us:
    
    https://fedoraproject.org/wiki/Changes/Removing_ldconfig_scriptlets
    
    These changes appear to be implemented for RHEL > 7 as well, so only
    run ldconfig on RHEL7
    
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Signed-off-by: Cole Robinson <crobinso@redhat.com>