Fix build for recent Cabal

[k-bx]

It currently fails with:

/home/kb/workspace/statvfs/Setup.hs:6:29: error:                                                                                    
    Variable not in scope: defaultUserHooks :: UserHooks                                                                            
  |                                                                                                                                 
6 | main = defaultMainWithHooks defaultUserHooks                                                                                    
  |                             ^^^^^^^^^^^^^^^^                                                                                    

[exarkun]

I encountered this error with Cabal 3.2.1.0. Once I also made autoreconf available on PATH (installed nixos autoconf package), this branch builds successfully for me.