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.

Source/cmWindowsRegistry.h

@@ -2,6 +2,7 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #pragma once
 
+#include <cstdint> // IWYU pragma: keep
 #include <string>
 #include <vector>