package/xen: Disable Werror checking

package/xen: Disable Werror checking

The strict unused-const-variable checking was causing autobuilder errors
when trying to build Xen tools/libxl as the migrate_*[] arrays are not
always accessed.

To avoid the error edit the Makefile to stop all general warnings being treated
as errors, by removing the -Werror flag.

Fixes:
  http://autobuild.buildroot.net/results/0e0/0e0d4aa4a05da5804821951289c0a4049b009c61/

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>