Unbreak the GHC build with older versions of gcc

Unbreak the GHC build with older versions of gcc
Patch from kili@outback.escape.de, who says:
  Stg.h must be included before HsBase.h, because the latter contains
  function definitions causing older versions of gcc (3.3.5 in my
  case) to bail out with "error: global register variable follows a
  function definition" on Regs.h, which is included by Stg.h.