Replace raw-type uses of Post with Post<?>.

Change-Id: I3ba2094ea9471d8d86af7c8d6a537d7eb43961de
diff --git a/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java b/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java
index 3a16868..f783ba0 100644
--- a/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java
+++ b/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java
@@ -413,7 +413,7 @@
     if (!(o instanceof Post)) {
       return false;
     }
-    Post post = (Post) o;
+    Post<?> post = (Post<?>) o;
     return Objects.equals(id, post.id);
   }