cmWindowsRegistry: Add missing <cstdint> include

It is needed for `std::uint8_t`, but was left out of commit 8d7e80cf3d
(find_* commands: add control over Windows registry views, 2022-04-16).
For some reason the include-what-you-use tool does not think the include
is needed, so add an `IWYU pragma` to keep it.