depends: always use correct ar for win qt

If we don't set this explicitly, then qt will still use it's default
windows ar, when building with LTO (when we want it to use gcc-ar).

So set `QMAKE_LIB` which is used for win32, and defaults to `ar -rc`.
This way we always get the correct ar.

Issue can be seen building in Guix with LTO. i.e:
```bash
x86_64-w64-mingw32-ar: .obj/release/hb-blob.o: plugin needed to handle lto object
```

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