virCgroupAddTaskStrController: s/-1/-ENOMEM/

Within whole vircgroup.c we 'return -errno', e.g. 'return -ENOMEM'.
However, in this specific function virCgroupAddTaskStrController
we weren't returning -ENOMEM but -1 despite fact that later in
the function we are returning one of errno values indeed.

src/util/vircgroup.c

@@ -1028,7 +1028,7 @@ static int virCgroupAddTaskStrController(virCgroupPtr group,
     char *endp;
 
     if (!(str = strdup(pidstr)))
-        return -1;
+        return -ENOMEM;
 
     cur = str;
     while (*cur != '\0') {