nscd: drop selinux/flask.h include

Building nscd w/selinux enabled yields a warning which yields an error:
In file included from selinux.c:32:0:
/usr/include/selinux/flask.h:5:2: error:
 #warning "Please remove any #include's of this header in your source code."

I've done just that and it builds cleanly with libselinux-2.4.