securityselinuxhelper.c: Don't try to include xattr.h if not available

securityselinuxhelper.c: Don't try to include xattr.h if not available

Commit 907a39e7 missed adding the WITH_ATTR around #include <attr/xattr.h>
resulting in a build failure:

  CC     libsecurityselinuxhelper_la-securityselinuxhelper.lo
  securityselinuxhelper.c:27:24: fatal error: attr/xattr.h: No such file or directory
  compilation terminated.
  make[2]: *** [libsecurityselinuxhelper_la-securityselinuxhelper.lo] Error 1
  make[2]: Leaving directory `/home/jferlan/libvirt.work/tests'
  make[1]: *** [all-recursive] Error 1
  make[1]: Leaving directory `/home/jferlan/libvirt.work'
  make: *** [all] Error 2