kbuild: do not add "selinux" to subdir- twice

scripts/Makefile adds "selinux" to subdir-y or subdir- twice.

  subdir-$(CONFIG_MODVERSIONS) += genksyms
  subdir-y                     += mod
  subdir-$(CONFIG_SECURITY_SELINUX) += selinux    <--- here
  subdir-$(CONFIG_DTC)         += dtc

  # Let clean descend into subdirs
  subdir- += basic kconfig package selinux        <--- again

The latter is redundant.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>