Update mighttpd2.cabal

Between 0.1.9 and 1.1.10
formatUnixTime went from:
formatUnixTime :: Format -> UnixTime -> ByteString
to
formatUnixTime :: Format -> UnixTime -> IO ByteString

Building with older versions of unix-time gives the error:
src/Report.hs:37:27:
    Couldn't match expected type `IO t0' with actual type `ByteString'
    Expected type: UnixTime -> IO t0
      Actual type: UnixTime -> ByteString
    In the return type of a call of `formatUnixTime'
    In the second argument of `(>>=)', namely
      `formatUnixTime "%d %b %Y %H:%M:%S"'
cabal: Error: some packages failed to install:
mighttpd2-2.8.6 failed during the building phase. The exception was:
ExitFailure 1