xen/cpupools: silence maybe-unitialized warning

When building with gcc with -finstrument-functions, optimization level
-O1, CONFIG_HYPFS=y and # CONFIG_HAS_SCHED_GRANULARITY is not set, the
the following build warning (error) is encountered:

common/sched/cpupool.c: In function ‘cpupool_gran_write’:
common/sched/cpupool.c:1220:26: error: ‘gran’ may be used uninitialized [-Werror=maybe-uninitialized]
 1220 |                      0 : cpupool_check_granularity(gran);
      |                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
common/sched/cpupool.c:1207:21: note: ‘gran’ declared here
 1207 |     enum sched_gran gran;
      |                     ^~~~

This is a false positive. Silence the warning (error) by initializing
the variable.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

xen/common/sched/cpupool.c

@@ -103,6 +103,7 @@ custom_param("sched-gran", sched_select_granularity);
 #elif defined(CONFIG_HYPFS)
 static int sched_gran_get(const char *str, enum sched_gran *mode)
 {
+    *mode = SCHED_GRAN_cpu;
     return -EINVAL;
 }
 #endif