FHSS unit test: fixed fhss beacon tasklet test

(cherry picked from commit abe6d671b058f4f069741eab24d51e4d62d550b0)

Author: Jarkko Paso

test/nanostack/unittest/service_libs/fhss_beacon_tasklet/Makefile

@@ -19,6 +19,7 @@ TEST_SRC_FILES = \
         ../../stub/fhss_platform_stub.c \
         ../../stub/fhss_mac_interface_stub.c \
         ../../stub/event_stub.c \
+        ../../stub/platform_stub.c \
 
 include ../../MakefileWorker.mk