mfplat.c pos undeclared

When trying to `make` the configured project i get this error:
```
dlls/mfreadwrite/tests/mfplat.c: In function ‘test_source_reader_from_media_source’:
dlls/mfreadwrite/tests/mfplat.c:977:5: error: ‘pos’ undeclared (first use in this function)
  977 |     pos.vt = VT_I8;
      |     ^~~
```