babl: use _aligned_malloc() instead of aligned_alloc() on Win32.

babl: use _aligned_malloc() instead of aligned_alloc() on Win32.

Thanks to Lionel N. for raising the issue. It was failing to build for
Windows with:
> babl/babl-space.c:503: undefined reference to `aligned_alloc'
Windows has apparently a similar API, except that the size and alignment
parameters are inverted.
See https://msdn.microsoft.com/library/8z34s9c6.aspx