build: [simple] Include qemu-timer-common.o in trace-obj-y

Helper programs like qemu-ga use tracing primitives, but qemu-timer-common.o
(also used by simpletrace.o) is not necessarily included in the linkage line.
Signed-off-by: NLluís Vilanova <vilanova@ac.upc.edu>

Makefile.objs

@@ -380,7 +380,7 @@ else
 trace-obj-y = trace.o
 ifeq ($(TRACE_BACKEND),simple)
 trace-obj-y += simpletrace.o
-user-obj-y += qemu-timer-common.o
+trace-obj-y += qemu-timer-common.o
 endif
 endif