selinux: fix Makefile dependencies of flask.h

Make the flask.h target depend on the genheaders binary instead of
classmap.h to ensure that it is rebuilt if any of the dependencies of
genheaders are changed.

Notably this fixes flask.h not being rebuilt when
initial_sid_to_string.h is modified.

Fixes: 8753f6be ("selinux: generate flask headers during kernel build")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>