Add xxhash-ffi to GHC 9.2 and 9.4 Nix configurations

Author: mrd

haskell-flake-ghc92.nix

@@ -12,6 +12,7 @@ pkgs: {
     singletons.source = "3.0.1"; # req because singletons-th-3.1 had bad bounds
     th-desugar.source = "1.13.1";
     th-abstraction.source = "0.4.5.0";
+    xxhash-ffi.source = "0.3";
   };
 
   # (note this is actually unused/we have to duplicate because it doesn't get

haskell-flake-ghc94.nix

@@ -12,6 +12,7 @@ pkgs: {
     #singletons.source = "3.0.1";
     th-desugar.source = "1.14";
     th-abstraction.source = "0.4.5.0";
+    xxhash-ffi.source = "0.3";
   };
 
   # (note this is actually unused/we have to duplicate because it doesn't get