FHSS unit test: fixed fhss mac interface test

(cherry picked from commit 805eb42e4416b00cc018dc32dceb353d0b6c8bb6)

Author: Jarkko Paso

test/nanostack/unittest/service_libs/fhss_mac_interface/Makefile

@@ -19,6 +19,7 @@ TEST_SRC_FILES = \
         ../../stub/fhss_callbacks_stub.c \
         ../../stub/fhss_beacon_tasklet_stub.c \
         ../../stub/randLIB_stub.c \
+        ../../stub/platform_stub.c \
 
 include ../../MakefileWorker.mk