Fix nativefiledialog port using build time include dir

[jfhs]

Describe the pull request

• What does your PR fix?

Fixes installed lib include path pointing to build-time location. Problem is in this bit:

target_include_directories(
    nfd
    PUBLIC
        $<BUILD_INTERFACE:
            ${CMAKE_CURRENT_LIST_DIR}/src
            ${CMAKE_CURRENT_LIST_DIR}/src/include
        >
        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
)

For install target, BUILD_INTERFACE evaluates to empty string, which is expanded to current build folder and so unofficial-nativefiledialog-config.cmake has lines like these that include absolute build folder path:

set_target_properties(unofficial::nativefiledialog::nfd PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "C:/src/vcpkg/buildtrees/nativefiledialog/src/44c798f8b9-0d908eb5b1.clean/;${_IMPORT_PREFIX}/include"
)

Separating into two different target_include_directories fixes it.

• Which triplets are supported/not supported? Have you updated the CI baseline?

  All (except UWP), No

• Does your PR follow the maintainer guide?

  Yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes

[ghost]

All CLA requirements met.

[vicroms]

Thanks for the PR!