Add patches hack to make ghc-8.6.4 builds work

Author: rvl

patches/default.nix

@@ -6,4 +6,5 @@
   ghc861 = import ./ghc861;
   ghc862 = import ./ghc862;
   ghc863 = import ./ghc863;
+  ghc864 = import ./ghc863;
 }