.pullapprove: Add selinux-maintainers

This project was initially approved as go-selinux [1] but has since
renamed itself to drop the go- prefix [2].

[1]: https://groups.google.com/a/opencontainers.org/forum/#!topic/tob/nH8QU6UnrqA
     Subject: VOTE Required: approve new project go-selinux
     Date: Fri, 17 Feb 2017 01:21:37 +0000
     Message-ID: <CAD2oYtPzgJKeiKKxFnds-yH+2OG3WF6d36gR1uD2Y3+QQdsgpw@mail.gmail.com>
[2]: opencontainers/selinux#6 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

.pullapprove.yml

@@ -40,3 +40,6 @@ groups:
   runtime-tools:
     teams:
       - runtime-tools-maintainers
+  selinux:
+    teams:
+      - selinux-maintainers