sim: cgen: move cgen-types.h include to cgen-defs.h

The cgen-types.h header sets up types that are needed by cgen-defs.h,
so move the include out of sim-main.h and to that header.  It might
be needed in other specific modules, but for now let's kick it out of
sim-main.h to make some progress.  Things still build with just this.