ifcparse: drop dllexport from instance_streamer class template

Fixes the following compilation error when building on MSVC with `USE_MMAP`.

```
parse.cpp(2202,23): error C2338: static_assert failed: 'Default instance_streamer requires a pushed sequential reader'
```

Apparently, when class has `IFC_PARSE_API`, compiler is very eager to instantiate all possible templates leading to this error. We already have explicit instantiation with export for the ones we actually use (e.g. `template class IFC_PARSE_API ifcopenshell::instance_streamer<file_reader<full_buffer_impl>>;`), so it should be no-op.

src/ifcparse/file.h

@@ -91,7 +91,7 @@ enum filetype {
 IFC_PARSE_API filetype guess_file_type(const std::string& path);
 
 template <typename Reader = file_reader<full_buffer_impl>>
-class IFC_PARSE_API instance_streamer {
+class instance_streamer {
 private:
     std::unique_ptr<Reader> owned_stream_;
     Reader* stream_;