pw_async2: Add IWYU export for dispatcher_base.h in dispatcher.h

I get a IWYU warning when I include dispatcher.h for types like
`pw::async2::Context`, because they are defined in dispatcher_base.h,
not dispatcher.h. However, there is a comment in the BUILD file that I
that I should't depend on //pw_async2:dispatcher_base directy:

https://cs.opensource.google/pigweed/pigweed/+/main:pw_async2/BUILD.bazel;l=55-56;drc=813fe272d52a81bbeb774a3d307086138db7bf8b

This CL adds a IWYU export pragma to the include of dispatcher_base.h
inside of dispatcher.h, to suppress the warning.

Change-Id: If73c68747db380cbd8a521eb93ce09e6db74c40b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/269515
Reviewed-by: Wyatt Hepler <hepler@google.com>
Docs-Not-Needed: Michael Brase <mbrase@google.com>
Commit-Queue: Michael Brase <mbrase@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>