Add containers to PACKAGE_STAGE0
May fix reported error on #ghc-devs:
20:31 <pparkkin> Any idea why I'm getting this when trying to make master?
20:31 <pparkkin> ghc/Main.hs:85:18: error: Could not load module ‘Data.Set’
20:32 <pparkkin> make[1]: *** [ghc/ghc.mk:118: ghc/stage1/build/.depend.haskell] Error 1
Since a6989971 the compiler depends on containers
package