Add missing translatable string

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/1681

src/_locales/en/messages.json

@@ -1145,6 +1145,10 @@
     "message": "Close this window",
     "description": "English: Close this window"
   },
+  "docblockedDontWarn": {
+    "message": "Don't warn me again for this site",
+    "description": "Label for checkbox in document-blocked page"
+  },
   "docblockedProceed": {
     "message": "Disable strict blocking for {{hostname}}",
     "description": "English: Disable strict blocking for {{hostname}} ..."