Use wildcards in 'extra-source-files'

Elm wasn't being caught.

I had changed to explicit paths when investigating rebuilds, but it was a red herring - I hadn't realised this fix was only in Cabal 3.4 and I'm using 3.2: haskell/cabal#6889.

Author: georgefst

haskell/monpad.cabal

@@ -5,9 +5,9 @@ license:             BSD-3-Clause
 author:              George Thomas
 maintainer:          [email protected]
 extra-source-files:
-    rsc/main.css
-    rsc/main.js
-    rsc/default.dhall
+    rsc/*.css
+    rsc/*.js
+    rsc/*.dhall
 
 flag release
     Description: Distributable build. Optimise, embed assets etc.