[Offload] Fix missing dependencies in Offload API generation (#142776)

Thanks to @RossBrunton for spotting this.

We attempt to clang-format the generated Offload header files, but if
clang-format isn't available we just copy the generated files instead.
That fallback path was missing the correct dependencies.

Fixes #142756

offload/liboffload/API/CMakeLists.txt

@@ -42,5 +42,7 @@ else()
             COMMAND ${CMAKE_COMMAND} -E copy_if_different generated/${file}.gen ${CMAKE_CURRENT_BINARY_DIR}/${file}
             DEPENDS generated/${file}.gen
         )
+        add_custom_target(OffloadAPI.${file} DEPENDS ${file})
+        add_dependencies(OffloadAPI OffloadAPI.${file})
     endforeach()
 endif()