Meta: Update GN build for recent changes

9a026fc8d5
ae1ac9871b
07b332e17c

And some missed changes from the past that weren't hit because no one
was referencing those symbols :^).

Meta/gn/secondary/Userland/Libraries/LibWeb/DOM/BUILD.gn

@@ -52,5 +52,6 @@ source_set("DOM") {
     "StyleElementUtils.cpp",
     "Text.cpp",
     "TreeWalker.cpp",
+    "XMLDocument.cpp",
   ]
 }