maint: cleanup detection of const'ness of selinux ctx
    
    Commit 292d3f2d fixed the build with libselinux 2.3, but missed
    some suggestions by eblake
    
    https://www.redhat.com/archives/libvir-list/2014-May/msg00977.html
    
    This patch changes the macro introduced in 292d3f2d to either be
    empty in the case of newer libselinux, or contain 'const' in the
    case of older libselinux.  The macro is then used directly in
    tests/securityselinuxhelper.c.