html: css fetcher: Fix passing LWC borrow to fetcher_add, which consumes.

Fixes abort on exit when corestring finalisation tried to unref
what should have been the final ref:

    $ ./nsgtk3
    corrupted double-linked list
    Aborted (core dumped)

content/handlers/html/html_css_fetcher.c

@@ -296,7 +296,8 @@ nserror html_css_fetcher_register(void)
 		.finalise = html_css_fetcher_finalise
 	};
 
-	return fetcher_add(corestring_lwc_x_ns_css, &html_css_fetcher_ops);
+	return fetcher_add(lwc_string_ref(corestring_lwc_x_ns_css),
+			&html_css_fetcher_ops);
 }
 
 /* exported interface documented in html_internal.h */