Nice error for global include of binder/Enum.h

Instead of "could not find header", if someone is relying on global
includes and using this new binder/AIDL feature, they will be instructed
how to switch to relying on libbinder headers.

Fixes: 169874616
Test: build only
Change-Id: I67ac5cfdbf088e244ad4f840045e785bf5b14e02