selinux: move genheaders to security/selinux/

This tool is only used in security/selinux/Makefile.

Move it to security/selinux/ so that 'make clean' can clean it up.

Please note 'make clean' does not clean scripts/ because tools under
scripts/ are often used for external module builds. Obviously, genheaders
is not the case here.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>