ftbfh: include cbits/decode.h in hackage package, fix cc error

'cabal install hadoop-formats' failed, missing decode.h

After adding decode.h to extra-source-files, the build failed with an error on initializers in for loops, requiring -std=c99.

[PATCH] cabal: include cbits/decode.h, build with -std=c99
Just curious how this is different to what was there before?