[PAL/Linux-SGX] Makefile: Let make clean work for sgx-driver

Pal/src/host/Linux-SGX/Makefile

@@ -101,6 +101,7 @@ CLEAN_FILES += $(ias_cert_file) quote/generated-cacert.h
 clean:
 	rm -rf *.o *.e *.i *.s $(host_files) $(CLEAN_FILES) *.d debugger/*.d signer/*.pyc __pycache__ \
 	       signer/__pycache__
+	$(MAKE) -C sgx-driver $@
 
 .PHONY: test
 test: