selinux: avoid extra semicolon

Wrap macro into `do { } while (0)` to avoid Clang emitting warnings
about extra semicolons.
Similar to userspace commit
https://github.com/SELinuxProject/selinux/commit/9d85aa60d12e468e7fd510c2b5475b5299b71622



Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: whitespace/indenting tweaks]
Signed-off-by: Paul Moore <paul@paul-moore.com>