trace: Only link generated-tracers.o with "simple" backend

In any other cases the object file is effectively empty, which is
disliked by ranlib and nm on Mac OS X.
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Tested-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

trace/Makefile.objs

@@ -140,8 +140,7 @@ $(obj)/generated-tcg-tracers.h-timestamp: $(SRC_PATH)/trace-events $(BUILD_DIR)/
 ######################################################################
 # Backend code
 
-util-obj-$(CONFIG_TRACE_SIMPLE) += simple.o
+util-obj-$(CONFIG_TRACE_SIMPLE) += simple.o generated-tracers.o
 util-obj-$(CONFIG_TRACE_FTRACE) += ftrace.o
 util-obj-$(CONFIG_TRACE_UST) += generated-ust.o
 util-obj-y += control.o
-util-obj-y += generated-tracers.o