licenses: Add asset entry for KaTeX license

We forgot to add that when we added the KaTeX fonts
and the LICENSE file in 829dae9.

Fixes: zulip#1540

Author: rajveermalviya

pubspec.yaml

@@ -115,6 +115,7 @@ flutter:
   uses-material-design: true
 
   assets:
+    - assets/KaTeX/LICENSE
     - assets/Noto_Color_Emoji/LICENSE
     - assets/Pygments/AUTHORS.txt
     - assets/Pygments/LICENSE.txt

test/licenses_test.dart

@@ -10,5 +10,5 @@ void main() {
   test('smoke: ensure all additional licenses load', () => awaitFakeAsync((async) async {
     await check(additionalLicenses().toList())
       .completes((it) => it.isNotEmpty());
-  }), skip: true); // TODO(#1540)
+  }));
 }