erp5_interface_post: add existing Post types to the interface_post type group

bt5/erp5_interface_post/PortalTypeTemplateItem/portal_types/Internet%20Message%20Post.xml

@@ -22,6 +22,14 @@
             <key> <string>factory</string> </key>
             <value> <string>addXMLObject</string> </value>
         </item>
+        <item>
+            <key> <string>group_list</string> </key>
+            <value>
+              <tuple>
+                <string>interface_post</string>
+              </tuple>
+            </value>
+        </item>
         <item>
             <key> <string>id</string> </key>
             <value> <string>Internet Message Post</string> </value>

bt5/erp5_interface_post/PortalTypeTemplateItem/portal_types/Letter%20Post.xml

@@ -22,6 +22,14 @@
             <key> <string>factory</string> </key>
             <value> <string>addXMLObject</string> </value>
         </item>
+        <item>
+            <key> <string>group_list</string> </key>
+            <value>
+              <tuple>
+                <string>interface_post</string>
+              </tuple>
+            </value>
+        </item>
         <item>
             <key> <string>id</string> </key>
             <value> <string>Letter Post</string> </value>