package/xen: fix build failure on x86 after bump to 4.7

package/xen: fix build failure on x86 after bump to 4.7

Since the bump to 4.7, Xen fails to build because of a double definition
of __OBJECT_FILE__. This is due to (who would have guessed) the weirdness
of their buildsystem.

Fix that with a dirty hack: undefine the macro before defining it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Alistair Francis <alistair.francis@xilinx.com>
[Alistair: Tested on ARMv7 (little endian) boot.]
Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>