Makefile: add missing deps on $(GENERATED_HEADERS)

This fixes a build issue with make -j6+ due to qapi-generated files
being built before $(GENERATED_HEADERS) have been created.

Tested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Makefile

@@ -192,8 +192,10 @@ test-qmp-commands.o: $(addprefix $(qapi-dir)/, test-qapi-types.c test-qapi-types
 test-qmp-commands: test-qmp-commands.o qfloat.o qint.o qdict.o qstring.o qlist.o qbool.o $(qapi-obj-y) error.o osdep.o qemu-malloc.o $(oslib-obj-y) qjson.o json-streamer.o json-lexer.o json-parser.o qerror.o qemu-error.o qemu-tool.o $(qapi-dir)/test-qapi-visit.o $(qapi-dir)/test-qapi-types.o $(qapi-dir)/test-qmp-marshal.o module.o
 
 QGALIB=qga/guest-agent-command-state.o qga/guest-agent-commands.o
+QGALIB_GEN=$(addprefix $(qapi-dir)/, qga-qapi-types.c qga-qapi-types.h qga-qapi-visit.c qga-qmp-marshal.c)
 
-qemu-ga.o: $(addprefix $(qapi-dir)/, qga-qapi-types.c qga-qapi-types.h qga-qapi-visit.c qga-qmp-marshal.c) $(qapi-obj-y)
+$(QGALIB_GEN): $(GENERATED_HEADERS)
+$(QGALIB) qemu-ga.o: $(QGALIB_GEN) $(qapi-obj-y)
 qemu-ga$(EXESUF): qemu-ga.o $(QGALIB) qemu-tool.o qemu-error.o error.o $(oslib-obj-y) $(trace-obj-y) $(block-obj-y) $(qobject-obj-y) $(version-obj-y) $(qapi-obj-y) qemu-timer-common.o qemu-sockets.o module.o qapi/qmp-dispatch.o qapi/qmp-registry.o $(qapi-dir)/qga-qapi-visit.o $(qapi-dir)/qga-qapi-types.o $(qapi-dir)/qga-qmp-marshal.o
 
 QEMULIBS=libhw32 libhw64 libuser libdis libdis-user