SELinux: rename selinux_netlabel.h to netlabel.h

In the beginning I named the file selinux_netlabel.h to avoid potential
namespace colisions.  However, over time I have realized that there are several
other similar cases of multiple header files with the same name so I'm changing
the name to something which better fits with existing naming conventions.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>