sched_rt: use the correct type for _cpumask_scratch

sched_rt: use the correct type for _cpumask_scratch

The commit 376bbbabbda607d2039b8f839f15ff02721597d2 "sched_rt: print useful
affinity info when dumping" breaks build on ARM64:

sched_rt.c: In function ‘rt_init’:
sched_rt.c:442:26: error: assignment from incompatible pointer type [-Werror]
         _cpumask_scratch = xmalloc_array(cpumask_var_t, nr_cpu_ids);
                          ^
sched_rt.c: In function ‘rt_alloc_pdata’:
sched_rt.c:489:29: error: passing argument 1 of ‘alloc_cpumask_var’ from incompatible pointer type [-Werror]
     if ( !alloc_cpumask_var(&_cpumask_scratch[cpu]) )

This is because cpumask_var_t is not a type alias to cpumask_t** when
the number of CPU > 2 * BITS_PER_LONG. The correct type for
_cpumask_scratch should be cpumask_var_t*.

Signed-off-by: Julien Grall <julien.grall@citrix.com>