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

Previously the events unit tests 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/equeue/CMakeLists.txt

@@ -23,7 +23,9 @@ target_sources(${TEST_NAME}
 
 target_link_libraries(${TEST_NAME}
     PRIVATE
-        mbed-headers
+        mbed-headers-base
+        mbed-headers-platform
+        mbed-headers-events
         mbed-stubs-events
         mbed-stubs-platform
         gmock_main