Fix build on GHC 6.12
Fix build on GHC 6.12

nurpax/aeson,jkarni/aeson,dmjio/aeson,sol/aeson,timmytofu/aeson,neobrain/aeson,23Skidoo/aeson,lykahb/aeson,roelvandijk/aeson,beni55/aeson,aelve/json-x,SeanRBurton/aeson,roelvandijk/aeson,plaprade/aeson,neobrain/aeson,sol/aeson,sol/aeson,JPMoresmau/aeson,abbradar/aeson,bwo/aeson