cmake: add find_dependency calls to config file

[iblech]

Gpt4all is a tool for locally running LLMs. It vendors (a modified version of an older version of) QXlsx. However, with current Qt, compilation of Gpt4all fails:

CMake Error at deps/QXlsx/QXlsx/CMakeLists.txt:167 (target_link_libraries):
  Target "QXlsx" links to:

    Qt6::GuiPrivate

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Using Claude Opus 4.5, @photm5 and I tracked this problem down to an issue in QXlsx. Unfortunately, we are not experts on Qt build infrastructure and cannot personally judge the appropriateness of this patch. However, we can confirm that Gpt4all builds and works nicely with it. Hence we submit this for your consideration :-) Thank you for your work on QXlsx!

Here is Claude's detailed description:

The generated QXlsxQt*Config.cmake was missing find_dependency() calls for Qt components. This causes build failures when consuming projects use find_package(QXlsxQt6) because Qt6::GuiPrivate (declared in INTERFACE_LINK_LIBRARIES) is not found.

Add find_dependency() for Core, Gui, and conditionally GuiPrivate (required for Qt 6.10+) following CMake package configuration best practices.

Fixes build failures with Qt 6.10+ where GuiPrivate became a separate CMake component (QTBUG-87776).