Why does ROS2 custom message colcon build give include file errors?

edit

I am new to ROS2, but very familiar with ROS1.

I am doing a very minor tweak of the ROS2 tutorial Creating custom ROS 2 msg and srv files and it fails to build. The base tutorial code works fine. I don't care about srv so I got rid of that. And I added a new message.

The new message I added is named Complex.msg, and is shown below

Header header
Num num

My CMakeLists.txt is as follows:

cmake_minimum_required(VERSION 3.8)
project(tutorial_interfaces)

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)
# uncomment the following section in order to fill in
# further dependencies manually.
# find_package(<dependency> REQUIRED)

find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Num.msg"
  "msg/Complex.msg"
 )

if(BUILD_TESTING)
  find_package(ament_lint_auto REQUIRED)
  # the following line skips the linter which checks for copyrights
  # uncomment the line when a copyright and license is not present in all source files
  #set(ament_cmake_copyright_FOUND TRUE)
  # the following line skips cpplint (only works in a git repo)
  # uncomment the line when this package is not in a git repo
  #set(ament_cmake_cpplint_FOUND TRUE)
  ament_lint_auto_find_test_dependencies()
endif()

ament_package()

And my package.xml is unchanged from default shown in the ROS2 galactic tutorial.

When I try to build this slightly tweaked tutorial I get build errors shown below. I suspect that my new message requires changes to CMakeLists.txt and/or package.xml. If so, can you please guide what changes I need.

Colcon build error

colcon build --packages-select tutorial_interfaces

Starting >>> tutorial_interfaces
--- stderr: tutorial_interfaces                             
In file included from .../colcon_ws/build/tutorial_interfaces/rosidl_generator_c/tutorial_interfaces/msg/detail/complex__functions.h:19,
                 from .../colcon_ws/build/tutorial_interfaces/rosidl_generator_c/tutorial_interfaces/msg/detail/complex__functions.c:4:
.../colcon_ws/build/tutorial_interfaces/rosidl_generator_c/tutorial_interfaces/msg/detail/complex__struct.h:22:10: fatal error: tutorial_interfaces/msg/detail/header__struct.h: No such file or directory
   22 | #include "tutorial_interfaces/msg/detail/header__struct.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

...More errors like above

Summary: 0 packages finished [3.30s]
  1 package failed: tutorial_interfaces
  1 package had stderr output: tutorial_interface