rts: Ensure that Rts.h is always included first

In general this is the convention that we use in the RTS. On Windows
things actually fail if we break it. For instance, you see things like:

   includes\stg\Types.h:26:9: error:
     warning: #warning "Mismatch between __USE_MINGW_ANSI_STDIO
     definitions. If using Rts.h make sure it is the first header
     included." [-Wcpp]