Use explicit type-casting in hlist_del() for C++ compatibility

/usr/local/include/osmocom/core/linuxlist.h:479:12: error: invalid conversion from ‘void*’ to ‘hlist_node*’ [-fpermissive]
  479 |  n->next = LLIST_POISON1;

Fixes: I8ef73a62fe9846ce45058eb21cf999dd3eed5741
Change-Id: I75b0a5fe097562007c53987d8d41811e9f35798d
diff --git a/include/osmocom/core/linuxlist.h b/include/osmocom/core/linuxlist.h
index 5ccd462..f28f88b 100644
--- a/include/osmocom/core/linuxlist.h
+++ b/include/osmocom/core/linuxlist.h
@@ -476,8 +476,8 @@
 static inline void hlist_del(struct hlist_node *n)
 {
 	__hlist_del(n);
-	n->next = LLIST_POISON1;
-	n->pprev = LLIST_POISON2;
+	n->next = (struct hlist_node *)LLIST_POISON1;
+	n->pprev = (struct hlist_node **)LLIST_POISON2;
 }
 
 /*! Delete the specified hlist_node from its list and initialize.