CMake: unit-tests: Make events stub only depend on headers it uses
CMake: unit-tests: Make events stub only depend on headers it uses

Previously the events stub library depended on `mbed-headers`, which
is a collection of all available headers in mbed-os. To make it easier
to separate the library, only depend on the headers we're using.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed