Add GHC 8.8.1 to tested-with stanza

Author: Ericson2314

reflex.cabal

@@ -18,7 +18,7 @@ extra-source-files:
   ChangeLog.md
 
 tested-with:
-  GHC  ==8.0.2 || ==8.2.2 || ==8.4.4 || ==8.6.5,
+  GHC  ==8.0.2 || ==8.2.2 || ==8.4.4 || ==8.6.5 || ==8.8.1,
   GHCJS ==8.4
 
 flag use-reflex-optimizer