sched/group_foreachchild.c: fix warning

fix compilation warning:

   group/group_foreachchild.c:85:10: warning: 'ret' may be used uninitialized [-Wmaybe-uninitialized]
   85 |   return ret;
      |          ^~~
   group/group_foreachchild.c:67:7: note: 'ret' was declared here
   67 |   int ret;

Signed-off-by: raiden00pl <raiden00@railab.me>

sched/group/group_foreachchild.c

@@ -64,7 +64,7 @@ int group_foreachchild(FAR struct task_group_s *group,
 {
   FAR sq_entry_t *curr;
   FAR sq_entry_t *next;
-  int ret;
+  int ret = OK;
 
   DEBUGASSERT(group);