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.

Author: rajkan01

events/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -30,6 +30,10 @@ target_include_directories(mbed-stubs-events
 
 target_link_libraries(mbed-stubs-events
     PRIVATE
-        mbed-headers
-        mbed-stubs-headers
+        mbed-headers-base
+        mbed-headers-drivers
+        mbed-headers-hal
+        mbed-headers-events
+        mbed-headers-rtos
+        mbed-headers-platform
 )