FHSS unit tests: Fixed missing platform stub

Author: Jarkko Paso

test/nanostack/unittest/service_libs/fhss_ws/Makefile

@@ -23,6 +23,7 @@ TEST_SRC_FILES = \
         ../../stub/fhss_common_stub.c \
         ../../stub/fhss_callbacks_stub.c \
         ../../stub/fhss_platform_stub.c \
+        ../../stub/platform_stub.c \
 
 include ../../MakefileWorker.mk