Fix CMAKE_INSTALL_PREFIX init behavior on Win32.

[ScottTodd]

Per the guidance at https://cmake.org/cmake/help/latest/variable/CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT.html, CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT can be "used by project code to change the default without overriding a user-provided value".

This allows users and integration projects like https://github.com/ROCm/TheRock to set an install path of their choosing while still defaulting to this project's preferred value if the value is not already set. For more context, see https://github.com/ROCm/TheRock/pull/340.

[ScottTodd]

Thanks for approving! Should I merge this myself with squash and merge, or do you want to do that yourself?

https://github.com/ROCm/hipFFT/blob/develop/.github/CONTRIBUTING.md#process

[evetsso]

We'll merge this but we've got a couple of other internal tasks to take care of first.