SLINT_TARGET_SOURCES sets target_include_directories using a generator expression

This ensures that the binary directory path isn't exposed in the
target's `INTERFACE_INCLUDE_DIRECTORIES`.

This prevents CMake errors when we `install` a target with generated
Slint sources in the include directory:

```
[cmake] CMake Error in slint/cpp/CMakeLists.txt:
[cmake]   Target "slint-demo-cpp" INTERFACE_INCLUDE_DIRECTORIES property contains
[cmake]   path:
[cmake]
[cmake]     "/path/to/build/slint/cpp"
[cmake]
[cmake]   which is prefixed in the build directory.
```

api/cpp/cmake/SlintMacro.cmake

@@ -91,5 +91,5 @@ function(SLINT_TARGET_SOURCES target)
 
         target_sources(${target} PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/${_SLINT_BASE_NAME}.h ${cpp_files})
     endforeach()
-    target_include_directories(${target} PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
+    target_include_directories(${target} PUBLIC "$<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>")
 endfunction()