stat_item.c: Fix compiler warnings

  CC       stat_item.lo
stat_item.c: In function 'osmo_stat_item_group_alloc': stat_item.c:84:28: warning: cast to pointer from integer of different
size [-Wint-to-pointer-cast]
   group->items[item_idx] = (void *)items_size;
                            ^
stat_item.c:98:24: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
    ((uint8_t *)items + (int)group->items[item_idx]);
                        ^
diff --git a/src/stat_item.c b/src/stat_item.c
index 0545ea0..a6a8481 100644
--- a/src/stat_item.c
+++ b/src/stat_item.c
@@ -52,7 +52,7 @@
 					    unsigned int idx)
 {
 	unsigned int group_size;
-	unsigned int items_size = 0;
+	unsigned long items_size = 0;
 	unsigned int item_idx;
 	void *items;
 
@@ -95,7 +95,7 @@
 	/* Update item pointers */
 	for (item_idx = 0; item_idx < desc->num_items; item_idx++) {
 		struct osmo_stat_item *item = (struct osmo_stat_item *)
-			((uint8_t *)items + (int)group->items[item_idx]);
+			((uint8_t *)items + (unsigned long)group->items[item_idx]);
 		unsigned int i;
 
 		group->items[item_idx] = item;