openharmony-standard-3.1: Add security/selinux git repository

This patch adds //base/security/selinux repository needed to build and
install //third_party/selinux:libselinux to the system image.

Since //third_party/selinux needs //third_party/FreeBSD only for the
source files fts.c and fts.h, this patch also adds a dependency on the
musl fts package and uses it instead of the source file from FreeBSD.

Signed-off-by: Thierry Escande <thierry.escande@huawei.com>

Merge me with