CMake: Move rtos stub headers to the rtos/tests/UNITTESTS/double directory

Move rtos stubs headers to the mbed-os/rtos/tests/UNITTESTS/double
directory and update CMake to create a mbed-stubs-rtos-headers
library for rtos stub headers

Author: rajkan01

rtos/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -1,6 +1,13 @@
 # Copyright (c) 2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
+add_library(mbed-stubs-rtos-headers INTERFACE)
+
+target_include_directories(mbed-stubs-rtos-headers
+    INTERFACE
+        .
+)
+
 add_library(mbed-stubs-rtos)
 
 add_definitions(-DUNITTEST)