Condition runST import in Data/Primitive/Array.hs

runST is not used in this file on GHC 7.2.x and higher, hence importing
it from Control.Monad.ST produces a warning during compilation.  This
commit makes the warning go away.
Just for the record: if `__GLASGOW_HASKELL__` was undefined, it's handled as if it was a `0`. If isn't a proper arithmetic expression comparable with `>=` et al.  CPP would fail with an error.