[MLton] Patch to mlton/codegen/x86-codegen/x86-translate.fun

[Matthew Fluet]

> Attempt to build MLton from trunk gives me the following error:
>
> Error: codegen/x86-codegen/x86-translate.fun 812.25.
>  Undefined structure Assembly.
>         parseAndElaborate raised in 7.88 + 5.91 (43% GC)
>
> Below is a trivial patch that seems to fix the problem.  Shall I
> commit it?

Yes it is the right fix; I went ahead and committed it.  In the future, 
feel free to commit.  SVN is there to roll back if need be.