Update +TCP submodule to committed header fix (#1399)

This was last updated to point to a PR reference.
This submodule needs to be updated to point to the
comitted commit.

manifest.yml

@@ -12,7 +12,7 @@ dependencies:
       path: "FreeRTOS/Source"
 
   - name: "FreeRTOS-Plus-TCP"
-    version: "1f1ffec"
+    version: "af6b379"
     repository:
       type: "git"
       url: "https://github.com/FreeRTOS/FreeRTOS-Plus-TCP.git"