selinux: more Makefile tweaks

A few small tweaks to improve the SELinux Makefile:

- Define a new variable, 'genhdrs', to represent both flask.h and
  av_permissions.h; this should help ensure consistent processing for
  both generated headers.

- Move the 'ccflags-y' variable closer to the top, just after the
  main 'obj-$(CONFIG_SECURITY_SELINUX)' definition to make it more
  visible and improve the grouping in the Makefile.

- Rework some of the vertical whitespace to improve some of the
  grouping in the Makefile.

Reviewed-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>