expat: update cmake build options

[Kwiboo]

This PR updates expat build options after the update to 2.2.9, build complains about:

-- Configuration
--   Prefix ..................... /usr
--   Build type ................. MinSizeRel
--   Shared libraries ........... ON
--   Character type ............. char (UTF-8)
--
--   Build documentation ........ OFF
--   Build examples ............. ON
--   Build fuzzers .............. OFF
--   Build tests ................ ON
--   Build tools (xmlwf) ........ ON
--   Install files .............. ON


CMake Warning:
  Manually-specified variables were not used by the project:

    BUILD_doc
    BUILD_examples
    BUILD_shared
    BUILD_tests
    BUILD_tools

With these changes expat now builds with:

-- Configuration
--   Prefix ..................... /usr
--   Build type ................. MinSizeRel
--   Shared libraries ........... ON
--   Character type ............. char (UTF-8)
--
--   Build documentation ........ OFF
--   Build examples ............. OFF
--   Build fuzzers .............. OFF
--   Build tests ................ OFF
--   Build tools (xmlwf) ........ OFF
--   Install files .............. ON

[MilhouseVH]

Thanks 😊