maint: cleanup detection of const'ness of selinux ctx

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.

(cherry picked from commit b109c097654c4fe003e8535481191f37a35d5d7b)