1.0.11.28: Fix one Win32 regression introduced by 1.0.11.26-27.

* grovel-headers.c now pulls in src/code/runtime.h, and so needs a
  kludge to keep Win32's definition of boolean from conflicting with
  SBCL's.  This kludge already existed in win32-os.c, so I moved it
  into runtime.h.