neverallow su_exec:file execute

Executing /system/xbin/su is only supported on userdebug builds
for a limited number of domains. On user builds, it should never
occur.

Add a compile time assertion (neverallow rule) that this is
always true.

Bug: 19647373
Change-Id: I231a438948ea2d47c1951207e117e0fb2728c532