selinux: ensure av_permissions.h is built when needed

The Makefile rule responsible for building flask.h and
av_permissions.h only lists flask.h as a target which means that
av_permissions.h is only generated when flask.h needs to be
generated.  This patch fixes this by adding av_permissions.h as a
target to the rule.

Fixes: 8753f6be ("selinux: generate flask headers during kernel build")
Signed-off-by: Paul Moore <paul@paul-moore.com>