open-code EFIVARFS_MAGIC macro if unavailable via header inclusion

Upstream kernel commit

  commit 91716322d834cba34f4a7ed5e4a39673eb90862b
  Author: Matt Fleming <matt.fleming@intel.com>
  Date:   Mon Oct 22 15:51:45 2012 +0100

      efivarfs: Add unique magic number

has not (yet) been backported to RHEL-6, as of
kernel-headers-2.6.32-358.18.1.el6.

Open-code the macro so that libefivar can be built on RHEL-6.4.z.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>