xen: sched: Fix Arm build after commit f855dd9625

xen: sched: Fix Arm build after commit f855dd9625

Commit f855dd9625 "sched: add minimalistic idle scheduler for free cpus"
introduce the use of ZERO_BLOCK_PTR in the scheduler code. However, the
define does not exist outside of xmalloc_tsf.c for non-x86 architecture.

This will result to a compilation error on Arm:

schedule.c: In function ‘sched_idle_alloc_vdata’:
schedule.c:100:12: error: ‘ZERO_BLOCK_PTR’ undeclared (first use in this function)
     return ZERO_BLOCK_PTR;
                 ^~~~~~~~~~~~~~
schedule.c:100:12: note: each undeclared identifier is reported only once for each function it appears in
schedule.c:101:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
cc1: all warnings being treated as errors

To avoid the compilation error, the default definition for
ZERO_BLOCK_PTR is now moved in xen/config.h allowing all the code to use
the define.

Fixes: f855dd9625 ('sched: add minimalistic idle scheduler for free cpus')
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>