Build runc with selinux support

docker-ce seems to be building runc with selinux support, let us follow
the same pattern here please:
https://github.com/docker/docker-ce/search?p=1&q=RUNC_BUILDTAGS&unscoped_q=RUNC_BUILDTAGS

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

README.md

@@ -102,7 +102,7 @@ testing purposes. The version tag carries the suffix "-TEST".*
 To add build tags to the make option the `BUILD_TAGS` variable must be set.
 
 ```bash
-make BUILD_TAGS='seccomp apparmor'
+make BUILD_TAGS='seccomp apparmor selinux'
 ```
 
 | Build Tag | Feature                            | Dependency                      |