Missing flag in pkgconfig file

Although not strictly necessary and functions without so far, the `-DUHDR_USING_SHARED_LIBRARY` hint might help the linker generate more optimal output on Windows.

When building a _shared_ lib on Windows (any compiler), the resulting .pc file should look like:
```
Cflags: -I${includedir} -DUHDR_USING_SHARED_LIBRARY
```
P.S. At least [on MSYS2 (using `pkgconf` rather than `pkg-config`), the same .pc file can also support both shared and static libs](https://www.msys2.org/docs/pkgconfig/#cflagsprivate-static-libraries) by also adding the (undocumented)
```
Cflags.private: -UUHDR_USING_SHARED_LIBRARY
```
as well.

See e.g. https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-libdeflate/002-pkg-config.patch

Edit: I see that vcpkg also ships `pkgconf` rather than `pkg-config`, so should work out there as well.