[CM] Cannot compile s7 on msvc

[jingkaimori]

This patch fixes build error

<bil at ccrma.stanford.edu> 于2023年1月1日周日 01:36写道：
>
> > warning C4013: 'lookup' undefined; assuming extern returning int
>
> I was trying to get rid of some #if's and forgot to include
> the lookup forward declaration in the msvc case; here's
> a change that ought to fix that problem:
>
> at line 4061, add the declaration
>
> static inline s7_pointer lookup(s7_scheme *sc, const s7_pointer symbol);
>
> Thanks for the bug report!
>