build: fix selinux.c build failure on 32 bit

build: fix selinux.c build failure on 32 bit

* src/selinux.c: Don't include the system "fts.h" as
that disallows _FILE_OFFSET_BITS=64 which gnulib auto enables
to support large files on 32 bit systems.  Instead include
our "xfts.h" which includes the less limited gnulib replacement,
and also a checked version of xfts_open().
(restorecon): Use the checked xfts_open() rather than the standard
fts_open().
Prompted by the continuous integration build failure at:
http://hydra.nixos.org/build/6934169