build: fix missing trace dep on GENERATED_HEADERS

fc764105 added an include for qemu-common.h to trace/control.h, which
made all users of this header file dependent on GENERATED_HEADERS. Since
it's used by pretty much all the trace backends now, make trace-obj-y
dependent on GENERATED_HEADERS.
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

Makefile.objs

@@ -390,6 +390,8 @@ trace-nested-y += control.o
 
 trace-obj-y += $(addprefix trace/, $(trace-nested-y))
 
+$(trace-obj-y): $(GENERATED_HEADERS)
+
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 # smartcard