build-sys: there is no qemu-ga.c

It got moved in qga/main.c from commit 2870dc34.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

Makefile

@@ -443,7 +443,7 @@ $(qapi-modules) $(SRC_PATH)/scripts/qapi-introspect.py $(qapi-py)
 		"GEN","$@")
 
 QGALIB_GEN=$(addprefix qga/qapi-generated/, qga-qapi-types.h qga-qapi-visit.h qga-qmp-commands.h)
-$(qga-obj-y) qemu-ga.o: $(QGALIB_GEN)
+$(qga-obj-y): $(QGALIB_GEN)
 
 qemu-ga$(EXESUF): $(qga-obj-y) $(COMMON_LDADDS)
 	$(call LINK, $^)