Add external-content subdomain to DuckDuckGo ruleset (#19466)

Author: Jochen-A-Fuerbacher

src/chrome/content/rules/DuckDuckGo.xml

@@ -41,6 +41,7 @@
 	<target host="bttf.duckduckgo.com" />
 	<target host="chrome.duckduckgo.com" />
 	<target host="dub.duckduckgo.com" />
+	<target host="external-content.duckduckgo.com" />
 	<target host="ff.duckduckgo.com" />
 	<target host="help.duckduckgo.com" />
 	<target host="i.duckduckgo.com" />