Fix CPP usage so fourmolu doesn't choke

[lehins]

Here is a list of files that trips up fourmolu:

> fourmolu --no-cabal -c -m inplace $(git ls-files -- '*.hs' | grep -v Setup.hs)
Loaded config from: /home/lehins/github/massiv/fourmolu.yaml
massiv/src/Data/Massiv/Array/Delayed/Push.hs:95:1-9
  The GHC parser (in Haddock mode) failed:
  parse error on input `mconcatDL'
massiv/src/Data/Massiv/Array/Manifest.hs:172:1
  The GHC parser (in Haddock mode) failed:
  parse error (possibly incorrect indentation or mismatched brackets)
massiv/src/Data/Massiv/Array/Manifest/Boxed.hs:795:1
  The GHC parser (in Haddock mode) failed:
  parse error (possibly incorrect indentation or mismatched brackets)