[DDMD] Make verror extern(C)

<del>This one is tricky.  In C++ or at least with dmc, va_list is a `char *`.  But in D/druntime, it's a `void *`.  This means the mangling is different for `verror` and it can't be used from whichever language it isn't implemented in.</del>

<del>Any better ideas?</del>

Ok, better idea - make it extern(C) so we don't have to care about mangling.