Unbreak cereal for GHC 7.6.3
Unbreak cereal for GHC 7.6.3

Before this change, it failed with:

Preprocessing library cereal-0.3.5.2...
src/Data/Serialize.hs:11:4:
     error: invalid preprocessing directive
      #-}
       ^
triplepointfive/cereal,triplepointfive/cereal,fpco/fpco-cereal,glguy/cereal,fpco/fpco-cereal,glguy/cereal